diff --git a/frontend/src/i18n/index.ts b/frontend/src/i18n/index.ts
index 7f2d303..4ed806d 100644
--- a/frontend/src/i18n/index.ts
+++ b/frontend/src/i18n/index.ts
@@ -1,7 +1,6 @@
 import i18n from 'i18next';
 import { initReactI18next } from 'react-i18next';
 import zhCN from '@/locales/zh-CN.json';
-import enUS from '@/locales/en-US.json';
 
 export type AppLanguage = 'zh-CN' | 'en-US';
 
@@ -12,6 +11,24 @@ export const SUPPORTED_LANGUAGES: { code: AppLanguage; label: string; flag: stri
 
 export const DEFAULT_LANGUAGE: AppLanguage = 'zh-CN';
 
+/** Track which locale resource bundles have been registered with i18next. */
+const loadedLocales = new Set<string>(['zh-CN']);
+
+/**
+ * Ensure a locale resource bundle is loaded before use.
+ * zh-CN is bundled inline (default/fallback language).
+ * en-US is lazy-loaded via dynamic import to reduce the main bundle by ~290 kB.
+ * Subsequent calls for an already-loaded locale are instant no-ops.
+ */
+export async function ensureLocaleLoaded(lang: string): Promise<void> {
+  if (loadedLocales.has(lang)) return;
+  if (lang === 'en-US') {
+    const mod = await import('@/locales/en-US.json');
+    i18n.addResourceBundle('en-US', 'translation', mod.default, true, true);
+    loadedLocales.add('en-US');
+  }
+}
+
 /** Read persisted language from localStorage (zustand persist key) */
 function getPersistedLanguage(): AppLanguage {
   try {
@@ -27,12 +44,15 @@ function getPersistedLanguage(): AppLanguage {
   return DEFAULT_LANGUAGE;
 }
 
+const persistedLang = getPersistedLanguage();
+
 i18n.use(initReactI18next).init({
   resources: {
     'zh-CN': { translation: zhCN },
-    'en-US': { translation: enUS },
   },
-  lng: getPersistedLanguage(),
+  // Start with zh-CN if the persisted language (en-US) hasn't been loaded yet.
+  // en-US is fetched asynchronously below and the language is switched once ready.
+  lng: persistedLang === 'en-US' ? 'zh-CN' : persistedLang,
   fallbackLng: 'zh-CN',
   interpolation: {
     escapeValue: false, // React already escapes
@@ -42,6 +62,15 @@ i18n.use(initReactI18next).init({
   },
 });
 
+// If the user previously selected en-US, load it asynchronously then switch.
+// There is a brief flash of Chinese before English loads — acceptable trade-off
+// for a ~290 kB reduction in the main entry chunk.
+if (persistedLang === 'en-US') {
+  void ensureLocaleLoaded('en-US').then(() => {
+    i18n.changeLanguage('en-US');
+  });
+}
+
 // Sync <html lang="..."> with current language
 i18n.on('languageChanged', (lng) => {
   if (typeof document !== 'undefined') {
diff --git a/frontend/src/store/index.ts b/frontend/src/store/index.ts
index 14b9db6..fcd3271 100644
--- a/frontend/src/store/index.ts
+++ b/frontend/src/store/index.ts
@@ -29,9 +29,10 @@ export const useLanguageStore = create<LanguageState>()(
     (set) => ({
       language: "zh-CN" as AppLanguage,
       setLanguage: (lang) => {
-        // 同步切换 i18next 实例
+        // Ensure the target locale bundle is loaded (en-US is lazy) before switching.
         import("@/i18n")
-          .then(({ default: i18n }) => {
+          .then(async ({ default: i18n, ensureLocaleLoaded }) => {
+            await ensureLocaleLoaded(lang);
             i18n.changeLanguage(lang);
           })
           .catch(() => {