diff --git a/pkg/storage/s3.go b/pkg/storage/s3.go
index 4ba6c772f..53d939dff 100644
--- a/pkg/storage/s3.go
+++ b/pkg/storage/s3.go
@@ -9,6 +9,7 @@ import (
 	"io"
 	"io/ioutil"
 	"net/url"
+	"path"
 	"regexp"
 	"strconv"
 	"strings"
@@ -279,8 +280,10 @@ func newS3Storage(backend *backup.S3, opts *ExternalStorageOptions) (*S3Storage,
 			return nil, errors.Annotatef(berrors.ErrStorageInvalidConfig, "Bucket %s is not accessible: %v", qs.Bucket, err)
 		}
 	}
+	if len(qs.Prefix) > 0 && !strings.HasSuffix(qs.Prefix, "/") {
+		qs.Prefix += "/"
+	}
 
-	qs.Prefix += "/"
 	return &S3Storage{
 		session: ses,
 		svc:     c,
@@ -380,28 +383,20 @@ func (rs *S3Storage) WalkDir(ctx context.Context, opt *WalkOption, fn func(strin
 	if opt == nil {
 		opt = &WalkOption{}
 	}
-
-	// NOTE: leave prefix empty if subDir is not set. Else, s3 will return empty result!
-	prefix := ""
-	if len(opt.SubDir) > 0 {
-		prefix = rs.options.Prefix + opt.SubDir
-		if len(prefix) > 0 && !strings.HasSuffix(prefix, "/") {
-			prefix += "/"
-		}
+	prefix := path.Join(rs.options.Prefix, opt.SubDir)
+	if len(prefix) > 0 && !strings.HasSuffix(prefix, "/") {
+		prefix += "/"
 	}
-
 	maxKeys := int64(1000)
 	if opt.ListCount > 0 {
 		maxKeys = opt.ListCount
 	}
-
 	req := &s3.ListObjectsInput{
 		Bucket:  aws.String(rs.options.Bucket),
+		Prefix:  aws.String(prefix),
 		MaxKeys: aws.Int64(maxKeys),
 	}
-	if len(prefix) > 0 {
-		req.Prefix = aws.String(prefix)
-	}
+
 	for {
 		// FIXME: We can't use ListObjectsV2, it is not universally supported.
 		// (Ceph RGW supported ListObjectsV2 since v15.1.0, released 2020 Jan 30th)
diff --git a/pkg/storage/s3_test.go b/pkg/storage/s3_test.go
index 38cab888c..31c61d34c 100644
--- a/pkg/storage/s3_test.go
+++ b/pkg/storage/s3_test.go
@@ -872,7 +872,7 @@ func (s *s3Suite) TestWalkDir(c *C) {
 			}, nil
 		}).
 		After(firstCall)
-	s.s3.EXPECT().
+	thirdCall := s.s3.EXPECT().
 		ListObjectsWithContext(ctx, gomock.Any()).
 		DoAndReturn(func(_ context.Context, input *s3.ListObjectsInput) (*s3.ListObjectsOutput, error) {
 			c.Assert(aws.StringValue(input.Marker), Equals, aws.StringValue(contents[3].Key))
@@ -883,6 +883,31 @@ func (s *s3Suite) TestWalkDir(c *C) {
 			}, nil
 		}).
 		After(secondCall)
+	fourthCall := s.s3.EXPECT().
+		ListObjectsWithContext(ctx, gomock.Any()).
+		DoAndReturn(func(_ context.Context, input *s3.ListObjectsInput) (*s3.ListObjectsOutput, error) {
+			c.Assert(aws.StringValue(input.Bucket), Equals, "bucket")
+			c.Assert(aws.StringValue(input.Prefix), Equals, "prefix/")
+			c.Assert(aws.StringValue(input.Marker), Equals, "")
+			c.Assert(aws.Int64Value(input.MaxKeys), Equals, int64(4))
+			c.Assert(aws.StringValue(input.Delimiter), Equals, "")
+			return &s3.ListObjectsOutput{
+				IsTruncated: aws.Bool(true),
+				Contents:    contents[:4],
+			}, nil
+		}).
+		After(thirdCall)
+	s.s3.EXPECT().
+		ListObjectsWithContext(ctx, gomock.Any()).
+		DoAndReturn(func(_ context.Context, input *s3.ListObjectsInput) (*s3.ListObjectsOutput, error) {
+			c.Assert(aws.StringValue(input.Marker), Equals, aws.StringValue(contents[3].Key))
+			c.Assert(aws.Int64Value(input.MaxKeys), Equals, int64(4))
+			return &s3.ListObjectsOutput{
+				IsTruncated: aws.Bool(false),
+				Contents:    contents[4:],
+			}, nil
+		}).
+		After(fourthCall)
 
 	// Ensure we receive the items in order.
 	i := 0
@@ -899,6 +924,22 @@ func (s *s3Suite) TestWalkDir(c *C) {
 	)
 	c.Assert(err, IsNil)
 	c.Assert(i, Equals, len(contents))
+
+	// test with empty subDir
+	i = 0
+	err = s.storage.WalkDir(
+		ctx,
+		&WalkOption{ListCount: 4},
+		func(path string, size int64) error {
+			comment := Commentf("index = %d", i)
+			c.Assert("prefix/"+path, Equals, *contents[i].Key, comment)
+			c.Assert(size, Equals, *contents[i].Size, comment)
+			i++
+			return nil
+		},
+	)
+	c.Assert(err, IsNil)
+	c.Assert(i, Equals, len(contents))
 }
 
 // TestWalkDirBucket checks WalkDir retrieves all directory content under a bucket.
@@ -929,7 +970,7 @@ func (s *s3SuiteCustom) TestWalkDirWithEmptyPrefix(c *C) {
 			Size: aws.Int64(27499),
 		},
 	}
-	s3API.EXPECT().
+	firstCall := s3API.EXPECT().
 		ListObjectsWithContext(ctx, gomock.Any()).
 		DoAndReturn(func(_ context.Context, input *s3.ListObjectsInput) (*s3.ListObjectsOutput, error) {
 			c.Assert(aws.StringValue(input.Bucket), Equals, "bucket")
@@ -942,6 +983,20 @@ func (s *s3SuiteCustom) TestWalkDirWithEmptyPrefix(c *C) {
 				Contents:    contents,
 			}, nil
 		})
+	s3API.EXPECT().
+		ListObjectsWithContext(ctx, gomock.Any()).
+		DoAndReturn(func(_ context.Context, input *s3.ListObjectsInput) (*s3.ListObjectsOutput, error) {
+			c.Assert(aws.StringValue(input.Bucket), Equals, "bucket")
+			c.Assert(aws.StringValue(input.Prefix), Equals, "sp/")
+			c.Assert(aws.StringValue(input.Marker), Equals, "")
+			c.Assert(aws.Int64Value(input.MaxKeys), Equals, int64(2))
+			c.Assert(aws.StringValue(input.Delimiter), Equals, "")
+			return &s3.ListObjectsOutput{
+				IsTruncated: aws.Bool(false),
+				Contents:    contents[:1],
+			}, nil
+		}).
+		After(firstCall)
 
 	// Ensure we receive the items in order.
 	i := 0
@@ -958,4 +1013,20 @@ func (s *s3SuiteCustom) TestWalkDirWithEmptyPrefix(c *C) {
 	)
 	c.Assert(err, IsNil)
 	c.Assert(i, Equals, len(contents))
+
+	// test with non-empty sub-dir
+	i = 0
+	err = storage.WalkDir(
+		ctx,
+		&WalkOption{SubDir: "sp", ListCount: 2},
+		func(path string, size int64) error {
+			comment := Commentf("index = %d", i)
+			c.Assert(path, Equals, *contents[i].Key, comment)
+			c.Assert(size, Equals, *contents[i].Size, comment)
+			i++
+			return nil
+		},
+	)
+	c.Assert(err, IsNil)
+	c.Assert(i, Equals, 1)
 }