diff --git a/src/state_portal.rs b/src/state_portal.rs
index fad2075..15eb327 100644
--- a/src/state_portal.rs
+++ b/src/state_portal.rs
@@ -198,9 +198,13 @@ impl StatePortal {
                     enc_height,
                     self.args.fps,
                 );
-                // 码率：未指定时按分辨率 × 帧率动态计算
+                // 码率:WebRTC 模式用保守默认(BWE 连接后立即覆盖),MP4 用公式
                 let actual_bitrate = self.args.bitrate.unwrap_or_else(|| {
-                    5 * (enc_width as u64) * (enc_height as u64) * (self.args.fps as u64) / 100
+                    if self.webrtc.is_some() {
+                        webrtc_startup_bitrate_bps(enc_width, enc_height)
+                    } else {
+                        5 * (enc_width as u64) * (enc_height as u64) * (self.args.fps as u64) / 100
+                    }
                 });
                 // GOP 大小：WebRTC 模式使用较大的 GOP（fps*2，最低20），MP4 模式使用 fps
                 let actual_gop_size = self.args.gop_size.unwrap_or_else(|| {
@@ -848,6 +852,22 @@ fn resolution_bitrate_bps(width: u32, height: u32, fps: u32) -> u64 {
     5 * u64::from(width) * u64::from(height) * u64::from(fps) / 100
 }
 
+/// Conservative startup bitrate for WebRTC mode, tier-based by total pixel count.
+/// BWE estimate arrives within milliseconds of client connect and overrides this;
+/// the startup value only affects the first IDR. See issue #21.
+fn webrtc_startup_bitrate_bps(width: u32, height: u32) -> u64 {
+    let pixels = u64::from(width) * u64::from(height);
+    if pixels <= 1_000_000 {
+        1_000_000
+    } else if pixels <= 2_500_000 {
+        2_000_000
+    } else if pixels <= 4_500_000 {
+        4_000_000
+    } else {
+        8_000_000
+    }
+}
+
 /// Select resolution tier based on BWE estimate.
 /// Returns (width, height) for the selected tier.
 fn select_resolution(current_w: u32, current_h: u32, bwe_bps: u64, fps: u32) -> (u32, u32) {
@@ -1026,6 +1046,14 @@ mod tests {
         assert_eq!(result, None);
     }
 
+    #[test]
+    fn webrtc_startup_bitrate_tiers_by_pixel_count() {
+        assert_eq!(webrtc_startup_bitrate_bps(1280, 720), 1_000_000);
+        assert_eq!(webrtc_startup_bitrate_bps(1920, 1080), 2_000_000);
+        assert_eq!(webrtc_startup_bitrate_bps(2560, 1440), 4_000_000);
+        assert_eq!(webrtc_startup_bitrate_bps(3840, 2160), 8_000_000);
+    }
+
     #[test]
     fn select_resolution_downscales_one_tier_below_sixty_percent() {
         let fps = 30;