diff --git a/app/config.py b/app/config.py
index a80d508..98a03ec 100644
--- a/app/config.py
+++ b/app/config.py
@@ -102,6 +102,8 @@ class WhiteboardConfig(BaseModel):
     heartbeat_miss_threshold: int = 5
     # board_id 合法字符集与长度上限，防路径/注入
     max_board_id_length: int = 64
+    # 单 board 并发连接上限，防资源耗尽（同 board 同时在线人数）
+    max_connections_per_board: int = 50
     # 列表/管理页分页默认值
     list_limit: int = 100
 
diff --git a/app/controllers/whiteboard_controller.py b/app/controllers/whiteboard_controller.py
index ae840dc..9be4490 100644
--- a/app/controllers/whiteboard_controller.py
+++ b/app/controllers/whiteboard_controller.py
@@ -124,7 +124,11 @@ async def whiteboard_ws(websocket: WebSocket, board_id: str) -> None:
 
     # 注册连接并下发 init
     conn = Connection(websocket=websocket, board_id=board_id, client_id=client_id)
-    await hub.register(conn)
+    ok = await hub.register(conn)
+    if not ok:
+        await _safe_send(websocket, {"type": "error", "msg": "该记事本在线人数已满"})
+        await _safe_close(websocket)
+        return
     await _safe_send(websocket, {
         "type": "init",
         "content": board.content,
@@ -133,9 +137,14 @@ async def whiteboard_ws(websocket: WebSocket, board_id: str) -> None:
     })
 
     # 主循环：收消息 -> 处理 -> 广播
+    # 单帧大小上限：与 content 限制对齐（256KB 文本 + JSON 开销，留余量到 512KB）
+    MAX_FRAME = 512 * 1024
     try:
         while True:
             raw = await websocket.receive_text()
+            if len(raw) > MAX_FRAME:
+                await _safe_send(websocket, {"type": "error", "msg": "消息过大"})
+                continue
             msg = _parse(raw)
             if msg is None:
                 continue
diff --git a/app/services/whiteboard_hub.py b/app/services/whiteboard_hub.py
index 5925089..1010c28 100644
--- a/app/services/whiteboard_hub.py
+++ b/app/services/whiteboard_hub.py
@@ -58,19 +58,26 @@ class WhiteboardHub:
         self.heartbeat_interval = cfg.heartbeat_interval_seconds
         self.heartbeat_miss_threshold = cfg.heartbeat_miss_threshold
         self.timeout_seconds = self.heartbeat_interval * self.heartbeat_miss_threshold
+        self.max_connections_per_board = cfg.max_connections_per_board
         # {board_id: set[Connection]}
         self._boards: dict[str, set[Connection]] = {}
         self._lock = asyncio.Lock()
 
     # ---------------- 连接生命周期 ----------------
 
-    async def register(self, conn: Connection) -> None:
-        """把已 accept 的连接加入 board 集合（WebSocket accept 由 controller 负责）。"""
+    async def register(self, conn: Connection) -> bool:
+        """把已 accept 的连接加入 board 集合。
+
+        返回 False 表示该 board 连接数已达上限（调用方应关闭连接）。
+        """
         async with self._lock:
             conns = self._boards.setdefault(conn.board_id, set())
+            if len(conns) >= self.max_connections_per_board:
+                return False
             conns.add(conn)
         logger.info("连接接入 board=%s client=%s（当前 %d 人）",
                     conn.board_id, conn.client_id, self.connection_count(conn.board_id))
+        return True
 
     async def disconnect(self, conn: Connection) -> None:
         """幂等移除连接；空 set 从 dict 删除以防内存泄漏。"""