diff --git a/frontend/src/router/RedirectIfAuthed.tsx b/frontend/src/router/RedirectIfAuthed.tsx
new file mode 100644
index 0000000..29d4823
--- /dev/null
+++ b/frontend/src/router/RedirectIfAuthed.tsx
@@ -0,0 +1,25 @@
+// REQ-USR-004: /login 守卫（BR2）。已登录访问登录页 → 回 from 或 /。
+import type { ReactNode } from 'react';
+import { Navigate, useLocation } from 'react-router-dom';
+import { useAppSelector } from '../store/hooks';
+
+interface RedirectIfAuthedProps {
+  children: ReactNode;
+}
+
+/**
+ * 已登录态判定（D：仅 token 即视为已登录，与 RequireAuth 的「token 存在即非未登录」语义对齐；
+ * 避免 token 已持久化、user 尚在拉取时登录页与守卫之间互相弹跳）。
+ * 已登录 → 重定向到 location.state.from（来源）或 /；否则渲染 children（LoginPage）。
+ */
+export default function RedirectIfAuthed({ children }: RedirectIfAuthedProps) {
+  const token = useAppSelector((s) => s.auth.token);
+  const location = useLocation();
+
+  if (token) {
+    const from = (location.state as { from?: string } | null)?.from;
+    return <Navigate to={from ?? '/'} replace />;
+  }
+
+  return <>{children}</>;
+}
diff --git a/frontend/tests/unit/RedirectIfAuthed.test.tsx b/frontend/tests/unit/RedirectIfAuthed.test.tsx
new file mode 100644
index 0000000..f1debfc
--- /dev/null
+++ b/frontend/tests/unit/RedirectIfAuthed.test.tsx
@@ -0,0 +1,60 @@
+// REQ-USR-004: RedirectIfAuthed —— 已登录访问 /login 回主页（BR2）
+import { describe, it, expect } from 'vitest';
+import { screen } from '@testing-library/react';
+import { Routes, Route } from 'react-router-dom';
+import { renderShell } from './renderShell';
+import RedirectIfAuthed from '../../src/router/RedirectIfAuthed';
+
+function LoginScreen() {
+  return <div data-testid="login-screen">login-screen</div>;
+}
+function HomeSentinel() {
+  return <div data-testid="home-sentinel">home-sentinel</div>;
+}
+function UsersSentinel() {
+  return <div data-testid="users-sentinel">users-sentinel</div>;
+}
+
+function renderTree(
+  initialEntries: { pathname: string; state?: unknown }[] | string[],
+  preloadedAuth?: Parameters<typeof renderShell>[1]['preloadedAuth'],
+) {
+  return renderShell(
+    <Routes>
+      <Route
+        path="/login"
+        element={
+          <RedirectIfAuthed>
+            <LoginScreen />
+          </RedirectIfAuthed>
+        }
+      />
+      <Route path="/" element={<HomeSentinel />} />
+      <Route path="/usr/users" element={<UsersSentinel />} />
+    </Routes>,
+    { initialEntries: initialEntries as never, preloadedAuth },
+  );
+}
+
+const READY_USER = {
+  token: 't',
+  user: { id: 1, sUserName: '朱子纯', sUserType: '超级管理员', sLanguage: '中文' },
+};
+
+describe('RedirectIfAuthed', () => {
+  it('renders children when unauthenticated', () => {
+    renderTree(['/login'], { token: null, user: null });
+    expect(screen.getByTestId('login-screen')).toBeInTheDocument();
+  });
+
+  it('redirects to / when already authenticated', () => {
+    renderTree(['/login'], READY_USER);
+    expect(screen.getByTestId('home-sentinel')).toBeInTheDocument();
+    expect(screen.queryByTestId('login-screen')).not.toBeInTheDocument();
+  });
+
+  it('redirects to from when present', () => {
+    renderTree([{ pathname: '/login', state: { from: '/usr/users' } }], READY_USER);
+    expect(screen.getByTestId('users-sentinel')).toBeInTheDocument();
+  });
+});