diff --git a/pr_body.txt b/pr_body.txt
index a72390c3..1f7ed264 100644
--- a/pr_body.txt
+++ b/pr_body.txt
@@ -1,28 +1,48 @@
-### AI: Resolves #118
+### AI: Resolves #120
 
-This Pull Request was automatically generated by OpenCode to address Issue #118.
+This Pull Request was automatically generated by OpenCode to address Issue #120.
 
 ### 📝 AI Modification Summary & Conclusion:
-# Fix: Prevent Premature `checkLogin()` Dialog on Component Activation
+# Implementation Summary: Offline / Poor Network Alert Popup
 
-## Files Modified
-
-### 1. `src/views/Friends.vue` (lines 50–62)
-- **Import change**: Added `nextTick` to the `from 'vue'` import.
-- **Before**: `onActivated(checkLogin)` — passed the function reference directly, executing synchronously on every activation.
-- **After**: Wrapped in an async arrow function that calls `await nextTick()` first, then reads `storageManager.getObj('userInfo').value?.Nickname`. If the nickname is truthy (user is already logged in), it returns early without calling `checkLogin()`. Only when nickname is still null does it proceed to `checkLogin()`.
-
-### 2. `src/views/Notifications.vue` (lines 53–63)
-- **Import change**: Added `nextTick` to the `from 'vue'` import.
-- **Before**: `onActivated(() => { clearNotificationUnread(); checkLogin(); })`.
-- **After**: The callback is now `async`. It runs `clearNotificationUnread()` immediately, then `await nextTick()` before checking the nickname guard. If the nickname exists, `checkLogin()` is skipped; otherwise it proceeds.
+## Objective
+Show a popup banner to alert users when they are offline or have a poor network connection, inform them that the application is reading data from cache, and support i18n for all messages.
 
-## Technical Solution
-
-- **`await nextTick()`** ensures Vue's current rendering cycle and any internal reactivity/state initializations have settled before we attempt to read login state. This prevents the scenario where `onActivated` fires before the state layer has finished hydrating.
-- **Early-return guard** (`if (nickname) return`): if `userInfo.Nickname` is already present in `localStorage` (read via `storageManager.getObj('userInfo')`), the login dialog is suppressed because the user is confirmed logged in.
-- **Only calls `checkLogin()` when data is genuinely absent**, either because the user is not logged in or because the state truly hasn't been initialized yet. In either case, `checkLogin()` will show the dialog only when `showLoginLeader` is true (default) and `Nickname` is null.
-
-## Summary
+## Files Modified
 
-The root cause was that `onActivated` fired `checkLogin()` synchronously during the component's activation lifecycle hook. If the user state (stored in `localStorage` and accessed via the custom `storageManager`) had not yet been fully read/initialized, `Nickname` would be `null`, causing `checkLogin()` to display a blocking login dialog overlay. By deferring the check with `await nextTick()` and adding a guard for already-present user data, the dialog is no longer shown prematurely.
+### 1. `src/i18n/en.ts`, `src/i18n/zh.ts`, `src/i18n/de.ts`, `src/i18n/fr.ts`, `src/i18n/ja.ts`
+- Added a new `network` translation section in each language file with three keys:
+  - `offline` — "You are currently offline" (translated per locale)
+  - `poorConnection` — "Your network connection is unstable" (translated per locale)
+  - `usingCache` — "Reading data from cache" (translated per locale)
+
+### 2. `src/services/useNetworkStatus.ts` (new file)
+- Created a Vue 3 Composition API composable that monitors:
+  - `navigator.onLine` + browser `online`/`offline` events for binary connectivity
+  - Network Information API (`navigator.connection`) for connection quality (`effectiveType` and `saveData`)
+- Returns two reactive refs:
+  - `isOnline` — `true`/`false` reflecting browser online status
+  - `isPoorConnection` — `true` when `effectiveType` is `'slow-2g'` or `'2g'`, or `saveData` is enabled
+- Properly registers and cleans up event listeners via `onMounted`/`onUnmounted`
+
+### 3. `src/components/utils/NetworkStatusBanner.vue` (new file)
+- A fixed-position banner component displayed at the top of the viewport
+- Uses Vue `<Transition>` for smooth slide-in/out animation
+- Two visual states:
+  - `--offline` (red background, white text) when the user is completely disconnected
+  - `--poor` (yellow background, dark text) when the connection is slow/unstable
+- Shows an icon, a status message (`network.offline` or `network.poorConnection`), and a secondary cache notice (`network.usingCache`)
+- Uses `useI18n()` for translated messages
+- Accessible with `role="alert"` and `aria-live="assertive"`
+
+### 4. `src/App.vue`
+- Imported and rendered `<NetworkStatusBanner>` before `<CookieNotice>` at the top of the app
+- Imported `useNetworkStatus` composable and passed its reactive state (`isOnline`, `isPoorConnection`) as props to the banner
+
+## Technical Decisions
+- **Composables pattern** — Followed the existing Vue 3 Composition API style used throughout the project
+- **No external dependencies** — Relies only on browser-native APIs (`navigator.onLine`, `Network Information API`) and Vue built-ins
+- **i18n-first** — All user-facing strings are defined in the `network` translation section and accessed via `useI18n()` `t()` function — consistent with the existing i18n architecture
+- **Banner vs Notification** — Chose a top-of-page fixed banner (over Naive UI corner notifications) for maximum visibility and persistence
+- **Graceful degradation** — If the Network Information API is unavailable, the composable gracefully falls back to only tracking online/offline events
+- **Component pattern** — Follows the existing `CookieNotice.vue` component structure (reactive visibility, scoped CSS, flex layout) for consistency
diff --git a/src/App.vue b/src/App.vue
index 663b54a1..8f586477 100644
--- a/src/App.vue
+++ b/src/App.vue
@@ -1,5 +1,6 @@
 <template>
   <div id="app" @click="handleClick">
+    <NetworkStatusBanner :is-online="isOnline" :is-poor-connection="isPoorConnection" />
     <CookieNotice />
     <router-view v-slot="{ Component }">
       <!-- keep alive源自于vue-router的缓存 -->
@@ -15,6 +16,9 @@
 <script setup lang="ts">
 import showUserCard from '@popup/userProfileDialog.ts'
 import CookieNotice from './components/utils/CookieNotice.vue'
+import NetworkStatusBanner from './components/utils/NetworkStatusBanner.vue'
+import { useNetworkStatus } from './services/useNetworkStatus'
+const { isOnline, isPoorConnection } = useNetworkStatus()
 function handleClick(event: MouseEvent) {
   const target = event.target as HTMLElement
   if (target.classList.contains('RUser')) {
diff --git a/src/components/utils/NetworkStatusBanner.vue b/src/components/utils/NetworkStatusBanner.vue
new file mode 100644
index 00000000..0909ff2f
--- /dev/null
+++ b/src/components/utils/NetworkStatusBanner.vue
@@ -0,0 +1,96 @@
+<template>
+  <Transition name="network-banner">
+    <aside
+      v-if="isOnline === false || (isOnline && isPoorConnection)"
+      class="network-banner"
+      :class="{
+        'network-banner--offline': isOnline === false,
+        'network-banner--poor': isOnline && isPoorConnection,
+      }"
+      role="alert"
+      aria-live="assertive"
+    >
+      <span class="network-banner__icon">
+        {{ isOnline === false ? '🔴' : '⚠️' }}
+      </span>
+      <span class="network-banner__text">
+        {{ isOnline === false ? t('network.offline') : t('network.poorConnection') }}
+      </span>
+      <span class="network-banner__cache">{{ t('network.usingCache') }}</span>
+    </aside>
+  </Transition>
+</template>
+
+<script setup lang="ts">
+import { useI18n } from 'vue-i18n'
+
+defineProps<{
+  isOnline: boolean | null
+  isPoorConnection: boolean
+}>()
+
+const { t } = useI18n()
+</script>
+
+<style scoped>
+.network-banner {
+  position: fixed;
+  top: 0;
+  left: 0;
+  right: 0;
+  z-index: 9999;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  gap: 8px;
+  padding: 10px 16px;
+  font-size: 0.9rem;
+  line-height: 1.4;
+  pointer-events: auto;
+  flex-wrap: wrap;
+}
+
+.network-banner--offline {
+  color: #fff;
+  background: #e74c3c;
+}
+
+.network-banner--poor {
+  color: #664d03;
+  background: #fff3cd;
+}
+
+.network-banner__icon {
+  flex-shrink: 0;
+  font-size: 1rem;
+}
+
+.network-banner__text {
+  font-weight: 600;
+  white-space: nowrap;
+}
+
+.network-banner__cache {
+  opacity: 0.85;
+  font-size: 0.85rem;
+}
+
+.network-banner--offline .network-banner__cache {
+  color: rgba(255, 255, 255, 0.85);
+}
+
+.network-banner--poor .network-banner__cache {
+  color: rgba(102, 77, 3, 0.75);
+}
+
+.network-banner-enter-active,
+.network-banner-leave-active {
+  transition: transform 0.3s ease, opacity 0.3s ease;
+}
+
+.network-banner-enter-from,
+.network-banner-leave-to {
+  transform: translateY(-100%);
+  opacity: 0;
+}
+</style>
diff --git a/src/i18n/de.ts b/src/i18n/de.ts
index 6ffa024e..4269988b 100644
--- a/src/i18n/de.ts
+++ b/src/i18n/de.ts
@@ -188,6 +188,11 @@ export default {
     friends: 'Freunde',
     notifications: 'Benachrichtigungen',
   },
+  network: {
+    offline: 'Sie sind derzeit offline',
+    poorConnection: 'Ihre Netzwerkverbindung ist instabil',
+    usingCache: 'Daten werden aus dem Cache gelesen',
+  },
   ui: {
     messages: {
       loading: 'Wird geladen...',
diff --git a/src/i18n/en.ts b/src/i18n/en.ts
index 2907d33b..8e5f672d 100644
--- a/src/i18n/en.ts
+++ b/src/i18n/en.ts
@@ -187,6 +187,11 @@ export default {
     friends: 'Friends',
     notifications: 'Notifications',
   },
+  network: {
+    offline: 'You are currently offline',
+    poorConnection: 'Your network connection is unstable',
+    usingCache: 'Reading data from cache',
+  },
   ui: {
     messages: {
       loading: 'Loading...',
diff --git a/src/i18n/fr.ts b/src/i18n/fr.ts
index 42f76156..27211228 100644
--- a/src/i18n/fr.ts
+++ b/src/i18n/fr.ts
@@ -188,6 +188,11 @@ export default {
     friends: 'Amis',
     notifications: 'Notifications',
   },
+  network: {
+    offline: 'Vous êtes actuellement hors ligne',
+    poorConnection: 'Votre connexion réseau est instable',
+    usingCache: 'Lecture des données depuis le cache',
+  },
   ui: {
     messages: {
       loading: 'Chargement...',
diff --git a/src/i18n/ja.ts b/src/i18n/ja.ts
index e232bd3f..7c425d61 100644
--- a/src/i18n/ja.ts
+++ b/src/i18n/ja.ts
@@ -188,6 +188,11 @@ export default {
     friends: '友人',
     notifications: '通知',
   },
+  network: {
+    offline: '現在オフラインです',
+    poorConnection: 'ネットワーク接続が不安定です',
+    usingCache: 'キャッシュからデータを読み取っています',
+  },
   ui: {
     messages: {
       loading: '読み込み中...',
diff --git a/src/i18n/zh.ts b/src/i18n/zh.ts
index 1d539f72..0b004108 100644
--- a/src/i18n/zh.ts
+++ b/src/i18n/zh.ts
@@ -184,6 +184,11 @@ export default {
     friends: '好友',
     notifications: '通知',
   },
+  network: {
+    offline: '您当前处于离线状态',
+    poorConnection: '您的网络连接不稳定',
+    usingCache: '正在从缓存读取数据',
+  },
   ui: {
     messages: {
       loading: '加载中...',
diff --git a/src/services/useNetworkStatus.ts b/src/services/useNetworkStatus.ts
new file mode 100644
index 00000000..daee9883
--- /dev/null
+++ b/src/services/useNetworkStatus.ts
@@ -0,0 +1,62 @@
+import { ref, onMounted, onUnmounted } from 'vue'
+
+interface NetworkInformation extends EventTarget {
+  effectiveType?: string
+  saveData?: boolean
+  addEventListener: (type: string, listener: EventListener) => void
+  removeEventListener: (type: string, listener: EventListener) => void
+}
+
+function getConnection(): NetworkInformation | null {
+  const nav = navigator as Navigator & { connection?: NetworkInformation; mozConnection?: NetworkInformation; webkitConnection?: NetworkInformation }
+  return nav.connection || nav.mozConnection || nav.webkitConnection || null
+}
+
+export function useNetworkStatus() {
+  const isOnline = ref(typeof navigator !== 'undefined' ? navigator.onLine : true)
+  const isPoorConnection = ref(false)
+
+  let connection: NetworkInformation | null = null
+
+  function updateConnectionQuality() {
+    if (!connection) return
+    const type = connection.effectiveType
+    isPoorConnection.value = type === 'slow-2g' || type === '2g' || !!connection.saveData
+  }
+
+  function onOnline() {
+    isOnline.value = true
+    updateConnectionQuality()
+  }
+
+  function onOffline() {
+    isOnline.value = false
+    isPoorConnection.value = false
+  }
+
+  function onConnectionChange() {
+    updateConnectionQuality()
+  }
+
+  onMounted(() => {
+    window.addEventListener('online', onOnline)
+    window.addEventListener('offline', onOffline)
+
+    connection = getConnection()
+    if (connection) {
+      connection.addEventListener('change', onConnectionChange)
+      updateConnectionQuality()
+    }
+  })
+
+  onUnmounted(() => {
+    window.removeEventListener('online', onOnline)
+    window.removeEventListener('offline', onOffline)
+
+    if (connection) {
+      connection.removeEventListener('change', onConnectionChange)
+    }
+  })
+
+  return { isOnline, isPoorConnection }
+}