diff --git a/backend/go.mod b/backend/go.mod
index 8ce6304..2d32a42 100644
--- a/backend/go.mod
+++ b/backend/go.mod
@@ -6,6 +6,7 @@ require (
 	github.com/gin-gonic/gin v1.12.0
 	github.com/golang-jwt/jwt/v5 v5.3.1
 	github.com/jackc/pgx/v5 v5.10.0
+	github.com/jackc/puddle/v2 v2.2.2
 	github.com/redis/go-redis/v9 v9.22.0
 	golang.org/x/crypto v0.56.0
 )
@@ -25,7 +26,6 @@ require (
 	github.com/goccy/go-yaml v1.19.2 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
-	github.com/jackc/puddle/v2 v2.2.2 // indirect
 	github.com/json-iterator/go v1.1.12 // indirect
 	github.com/klauspost/cpuid/v2 v2.3.0 // indirect
 	github.com/leodido/go-urn v1.4.0 // indirect
diff --git a/backend/internal/api/api.go b/backend/internal/api/api.go
index 3fe1740..b4ab45b 100644
--- a/backend/internal/api/api.go
+++ b/backend/internal/api/api.go
@@ -1,10 +1,17 @@
 package api
 
 import (
+	"errors"
+	"io"
+	"log"
+	"net"
 	"net/http"
 	"strings"
+	"syscall"
 
 	"github.com/gin-gonic/gin"
+	"github.com/jackc/pgx/v5/pgconn"
+	"github.com/jackc/puddle/v2"
 
 	"booklib/internal/auth"
 	"booklib/internal/config"
@@ -24,6 +31,21 @@ func err(c *gin.Context, status int, code, msg string) {
 	c.AbortWithStatusJSON(status, gin.H{"error": gin.H{"code": code, "message": msg}})
 }
 
+// dbErr 统一处理 store 层失败:记日志;连接类错误 503(Service Unavailable),其余 500
+// 注:brief 里的 pgxpool.ErrClosedPool 在 pgx v5 不存在,实际由 puddle 原样透出,用它替代;
+// PG 停机时池内连接先收到 SQLSTATE 57P01(administrator shutdown),故把 08xx/57Pxx 也归为 503
+func dbErr(c *gin.Context, e error) {
+	log.Printf("db: %v", e)
+	status, code := http.StatusInternalServerError, "internal"
+	var pgErr *pgconn.PgError
+	connClass := errors.As(e, &pgErr) && (strings.HasPrefix(pgErr.Code, "08") || strings.HasPrefix(pgErr.Code, "57P"))
+	if connClass || errors.Is(e, syscall.ECONNREFUSED) || errors.Is(e, io.ErrUnexpectedEOF) ||
+		errors.Is(e, net.ErrClosed) || errors.Is(e, puddle.ErrClosedPool) {
+		status, code = http.StatusServiceUnavailable, "unavailable"
+	}
+	err(c, status, code, "db error")
+}
+
 func (a *api) authMw() gin.HandlerFunc {
 	return func(c *gin.Context) {
 		h := c.GetHeader("Authorization")
diff --git a/backend/internal/api/auth.go b/backend/internal/api/auth.go
index d793521..4330d37 100644
--- a/backend/internal/api/auth.go
+++ b/backend/internal/api/auth.go
@@ -2,7 +2,6 @@ package api
 
 import (
 	"errors"
-	"log"
 	"net/http"
 	"time"
 
@@ -28,8 +27,7 @@ func (a *api) login(c *gin.Context) {
 	u, qerr := a.st.GetUserByName(c, req.Username)
 	if qerr != nil {
 		if !errors.Is(qerr, pgx.ErrNoRows) {
-			log.Printf("db: %v", qerr)
-			err(c, http.StatusInternalServerError, "internal", "db error")
+			dbErr(c, qerr)
 			return
 		}
 		// 用户不存在也走一次 bcrypt，防用户名枚举时序差
diff --git a/backend/internal/api/books.go b/backend/internal/api/books.go
index c921bca..65a6bd6 100644
--- a/backend/internal/api/books.go
+++ b/backend/internal/api/books.go
@@ -24,7 +24,7 @@ func (a *api) getBookRow(c *gin.Context, id int64) (store.Book, bool) {
 			err(c, http.StatusNotFound, "not_found", "no such book")
 			return store.Book{}, false
 		}
-		err(c, http.StatusInternalServerError, "internal", "db error")
+		dbErr(c, e)
 		return store.Book{}, false
 	}
 	return b, true
@@ -87,7 +87,7 @@ func (a *api) listBooks(c *gin.Context) {
 	libID, _ := strconv.ParseInt(c.Query("library"), 10, 64)
 	views, e := a.st.ListBooks(c, libID, c.Query("q"), c.Query("prefix"), uid(c))
 	if e != nil {
-		err(c, http.StatusInternalServerError, "internal", "db error")
+		dbErr(c, e)
 		return
 	}
 	out := make([]gin.H, 0, len(views))
@@ -133,7 +133,7 @@ func (a *api) deleteBook(c *gin.Context) {
 	os.RemoveAll(bookfile.CoverDir(a.cfg.CacheDir, key))
 	os.RemoveAll(bookfile.PagesDir(a.cfg.CacheDir, key))
 	if e := a.st.DeleteBook(c, b.ID); e != nil {
-		err(c, http.StatusInternalServerError, "internal", "db error")
+		dbErr(c, e)
 		return
 	}
 	c.Status(http.StatusNoContent)
diff --git a/backend/internal/api/libraries.go b/backend/internal/api/libraries.go
index 171f889..1077587 100644
--- a/backend/internal/api/libraries.go
+++ b/backend/internal/api/libraries.go
@@ -3,7 +3,6 @@ package api
 import (
 	"context"
 	"io"
-	"log"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -31,8 +30,7 @@ func (a *api) libRoot(c *gin.Context, lib store.Library) (string, bool) {
 func (a *api) listLibraries(c *gin.Context) {
 	libs, e := a.st.ListLibraries(c)
 	if e != nil {
-		log.Printf("db: %v", e)
-		err(c, http.StatusInternalServerError, "internal", "db error")
+		dbErr(c, e)
 		return
 	}
 	out := make([]gin.H, 0, len(libs))
diff --git a/backend/internal/api/progress.go b/backend/internal/api/progress.go
index 9aa7337..2252876 100644
--- a/backend/internal/api/progress.go
+++ b/backend/internal/api/progress.go
@@ -33,7 +33,7 @@ func (a *api) putProgress(c *gin.Context) {
 		return
 	}
 	if e := a.st.UpsertProgress(c, uid(c), b.LibraryID, b.Path, req.Locator, req.Percent); e != nil {
-		err(c, http.StatusInternalServerError, "internal", "db error")
+		dbErr(c, e)
 		return
 	}
 	c.Status(http.StatusNoContent)
@@ -42,7 +42,7 @@ func (a *api) putProgress(c *gin.Context) {
 func (a *api) listProgress(c *gin.Context) {
 	rows, e := a.st.ListProgress(c, uid(c))
 	if e != nil {
-		err(c, http.StatusInternalServerError, "internal", "db error")
+		dbErr(c, e)
 		return
 	}
 	out := make([]gin.H, 0, len(rows))
diff --git a/backend/internal/api/router_test.go b/backend/internal/api/router_test.go
index 3ca7450..6eb9c1f 100644
--- a/backend/internal/api/router_test.go
+++ b/backend/internal/api/router_test.go
@@ -1,11 +1,18 @@
 package api
 
 import (
+	"errors"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"syscall"
 	"testing"
 	"time"
 
+	"github.com/gin-gonic/gin"
+	"github.com/jackc/pgx/v5/pgconn"
+	"github.com/jackc/puddle/v2"
+
 	"booklib/internal/config"
 	"booklib/internal/redispkg"
 )
@@ -23,3 +30,23 @@ func TestHealthz(t *testing.T) {
 		t.Fatalf("healthz = %d, want 200", w.Code)
 	}
 }
+
+func TestDBErrStatus(t *testing.T) {
+	for _, tc := range []struct {
+		e    error
+		want int
+	}{
+		{&pgconn.PgError{Code: "57P01"}, http.StatusServiceUnavailable}, // PG 停机:池内连接被服务端断
+		{&pgconn.PgError{Code: "08006"}, http.StatusServiceUnavailable},
+		{fmt.Errorf("dial: %w", syscall.ECONNREFUSED), http.StatusServiceUnavailable},
+		{puddle.ErrClosedPool, http.StatusServiceUnavailable},
+		{errors.New("boom"), http.StatusInternalServerError},
+	} {
+		w := httptest.NewRecorder()
+		c, _ := gin.CreateTestContext(w)
+		dbErr(c, tc.e)
+		if w.Code != tc.want {
+			t.Errorf("dbErr(%v) = %d, want %d", tc.e, w.Code, tc.want)
+		}
+	}
+}
diff --git a/backend/internal/api/users.go b/backend/internal/api/users.go
index 2778d90..c589d57 100644
--- a/backend/internal/api/users.go
+++ b/backend/internal/api/users.go
@@ -2,7 +2,6 @@ package api
 
 import (
 	"errors"
-	"log"
 	"net/http"
 	"strconv"
 	"time"
@@ -22,8 +21,7 @@ func isUnique(e error) bool {
 func (a *api) listUsers(c *gin.Context) {
 	users, e := a.st.ListUsers(c)
 	if e != nil {
-		log.Printf("db: %v", e)
-		err(c, http.StatusInternalServerError, "internal", "db error")
+		dbErr(c, e)
 		return
 	}
 	out := make([]gin.H, 0, len(users))
@@ -59,7 +57,7 @@ func (a *api) createUser(c *gin.Context) {
 			err(c, http.StatusConflict, "exists", "username taken")
 			return
 		}
-		err(c, http.StatusBadRequest, "bad_request", "invalid input")
+		dbErr(c, e)
 		return
 	}
 	c.JSON(http.StatusCreated, gin.H{"id": id, "username": req.Username, "role": req.Role})
@@ -81,7 +79,7 @@ func (a *api) deleteUser(c *gin.Context) {
 			err(c, http.StatusNotFound, "not_found", "no such user")
 			return
 		}
-		err(c, http.StatusInternalServerError, "internal", "db error")
+		dbErr(c, e)
 		return
 	}
 	if target.Role == "admin" {
@@ -92,7 +90,7 @@ func (a *api) deleteUser(c *gin.Context) {
 		}
 	}
 	if e := a.st.DeleteUser(c, id); e != nil {
-		err(c, http.StatusInternalServerError, "internal", "db error")
+		dbErr(c, e)
 		return
 	}
 	c.Status(http.StatusNoContent)