diff --git a/server/internal/agentd/singbox.go b/server/internal/agentd/singbox.go
index 5e9b38b..70f5969 100644
--- a/server/internal/agentd/singbox.go
+++ b/server/internal/agentd/singbox.go
@@ -15,11 +15,12 @@ import (
 	agentv1 "github.com/wangjia/pangolin/server/internal/pb/agentv1"
 )
 
-// Restarter applies a freshly-rendered sing-box config. sing-box has no hot
-// reload, so the production implementation restarts the systemd unit (a ~1-2s
-// data-plane blip). Tests inject a fake.
+// Restarter applies a freshly-rendered sing-box config. sing-box 支持 SIGHUP
+// 热重载(进程内 validate→close 旧→start 新,坏配置保留旧实例不断网),故配置
+// 变更走 Reload(不整进程重启、不全断流),冷启动走 Restart。Tests inject a fake.
 type Restarter interface {
-	Restart(ctx context.Context) error
+	Restart(ctx context.Context) error // 冷启动:systemctl restart(~1-2s blip)
+	Reload(ctx context.Context) error  // 热重载:SIGHUP 给 sing-box MainPID
 }
 
 // Cred is the agent's in-memory view of one data-plane credential. It is the
@@ -60,6 +61,11 @@ type SingBox struct {
 	hy2           *agentv1.Hy2Inbound
 	configVersion int64
 
+	// started 标记 sing-box 是否已被本 agent 冷启动过:首次走 Restart(冷启动),
+	// 之后配置变更走 Reload(SIGHUP 热重载)。agent 进程重启后复位为 false,
+	// 下次渲染做一次冷启动以确保与渲染配置一致。
+	started bool
+
 	// debounce machinery
 	dirty   chan struct{}
 	done    chan struct{}
@@ -328,7 +334,21 @@ func (s *SingBox) writeAndRestart(ctx context.Context) error {
 	if err := atomicWrite(s.cfg.SingboxConfigPath, data, 0o644); err != nil {
 		return err
 	}
-	return s.restarter.Restart(ctx)
+	// 首次/进程未被本 agent 启动过 → 冷启动;已在跑 + 配置变更 → SIGHUP 热重载
+	// (Reload 内部在取不到 PID/发信号失败时已自动回退 Restart)。
+	s.mu.Lock()
+	started := s.started
+	s.mu.Unlock()
+	if !started {
+		if err := s.restarter.Restart(ctx); err != nil {
+			return err
+		}
+		s.mu.Lock()
+		s.started = true
+		s.mu.Unlock()
+		return nil
+	}
+	return s.restarter.Reload(ctx)
 }
 
 // ensureHy2Cert 为启用 hy2 的节点准备自签证书(幂等),并把 CertPath/KeyPath 写回当前
@@ -396,3 +416,4 @@ func (s *SingBox) Run(ctx context.Context) {
 type noopRestarter struct{}
 
 func (noopRestarter) Restart(context.Context) error { return nil }
+func (noopRestarter) Reload(context.Context) error  { return nil }
diff --git a/server/internal/agentd/singbox_test.go b/server/internal/agentd/singbox_test.go
index 209ee9c..e70b33d 100644
--- a/server/internal/agentd/singbox_test.go
+++ b/server/internal/agentd/singbox_test.go
@@ -15,8 +15,9 @@ import (
 
 // fakeRestarter counts Restart calls.
 type fakeRestarter struct {
-	mu sync.Mutex
-	n  int
+	mu      sync.Mutex
+	n       int // Restart 次数(冷启动)
+	reloads int // Reload 次数(热重载)
 }
 
 func (f *fakeRestarter) Restart(context.Context) error {
@@ -26,12 +27,25 @@ func (f *fakeRestarter) Restart(context.Context) error {
 	return nil
 }
 
+func (f *fakeRestarter) Reload(context.Context) error {
+	f.mu.Lock()
+	f.reloads++
+	f.mu.Unlock()
+	return nil
+}
+
 func (f *fakeRestarter) count() int {
 	f.mu.Lock()
 	defer f.mu.Unlock()
 	return f.n
 }
 
+func (f *fakeRestarter) reloadCount() int {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	return f.reloads
+}
+
 func testConfig(t *testing.T) Config {
 	t.Helper()
 	dir := t.TempDir()
@@ -227,3 +241,26 @@ func assertNoIdentityFields(t *testing.T, blob []byte) {
 		}
 	}
 }
+
+// 首次渲染走冷启动(Restart),之后的配置变更走 SIGHUP 热重载(Reload)。
+func TestWriteAndRestartFirstColdThenReload(t *testing.T) {
+	fr := &fakeRestarter{}
+	sb := NewSingBox(testConfig(t), fr)
+	sb.ApplyConfig(sampleSnapshot(), true) // 设置 inbounds 使渲染成功
+	ctx := context.Background()
+
+	if err := sb.writeAndRestart(ctx); err != nil {
+		t.Fatalf("first writeAndRestart: %v", err)
+	}
+	if r, rl := fr.count(), fr.reloadCount(); r != 1 || rl != 0 {
+		t.Fatalf("首次后 restarts=%d reloads=%d, 期望 1/0", r, rl)
+	}
+
+	sb.Upsert(&Cred{DpUUID: "z", Protocol: agentv1.ProtocolBoth})
+	if err := sb.writeAndRestart(ctx); err != nil {
+		t.Fatalf("second writeAndRestart: %v", err)
+	}
+	if r, rl := fr.count(), fr.reloadCount(); r != 1 || rl != 1 {
+		t.Fatalf("变更后 restarts=%d reloads=%d, 期望 1/1(不再 Restart,走 Reload)", r, rl)
+	}
+}
diff --git a/server/internal/agentd/util.go b/server/internal/agentd/util.go
index d2019cf..2d7e5f9 100644
--- a/server/internal/agentd/util.go
+++ b/server/internal/agentd/util.go
@@ -7,6 +7,9 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"strconv"
+	"strings"
+	"syscall"
 )
 
 // logf is the agent's structured-ish logger. It deliberately never logs
@@ -48,15 +51,44 @@ type SystemdRestarter struct {
 	Unit string // e.g. "sing-box"
 }
 
-// Restart runs `systemctl restart <unit>`.
+// Restart runs `systemctl restart <unit>` (cold start: ~1-2s data-plane blip).
 func (r SystemdRestarter) Restart(ctx context.Context) error {
-	unit := r.Unit
-	if unit == "" {
-		unit = "sing-box"
-	}
+	unit := r.unit()
 	cmd := exec.CommandContext(ctx, "systemctl", "restart", unit)
 	if out, err := cmd.CombinedOutput(); err != nil {
 		return fmt.Errorf("agentd: restart %s: %w: %s", unit, err, out)
 	}
 	return nil
 }
+
+// Reload hot-reloads sing-box via SIGHUP to its MainPID. sing-box validates the
+// new config first and keeps the OLD instance running on error (no full
+// process restart). agent 与 sing-box 同 pangolin 用户,直接发信号即可(免
+// polkit/不改 unit)。PID 取不到或发信号失败时回退到 Restart(冷启动)。
+func (r SystemdRestarter) Reload(ctx context.Context) error {
+	unit := r.unit()
+	pid, err := singboxMainPID(ctx, unit)
+	if err != nil || pid <= 0 {
+		return r.Restart(ctx)
+	}
+	if err := syscall.Kill(pid, syscall.SIGHUP); err != nil {
+		return r.Restart(ctx)
+	}
+	return nil
+}
+
+func (r SystemdRestarter) unit() string {
+	if r.Unit == "" {
+		return "sing-box"
+	}
+	return r.Unit
+}
+
+// singboxMainPID 读 systemd 记录的主进程 PID(0 表示未在跑)。
+func singboxMainPID(ctx context.Context, unit string) (int, error) {
+	out, err := exec.CommandContext(ctx, "systemctl", "show", "-p", "MainPID", "--value", unit).Output()
+	if err != nil {
+		return 0, err
+	}
+	return strconv.Atoi(strings.TrimSpace(string(out)))
+}