diff --git a/CHANGELOG.md b/CHANGELOG.md
index 410cb364b20..0a38750d329 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@
 1. [20754](https://github.com/influxdata/influxdb/pull/20754): Update references to docs site to use current URLs.
 1. [20773](https://github.com/influxdata/influxdb/pull/20773): Fix data race in TSM engine when inspecting tombstone stats.
 1. [20797](https://github.com/influxdata/influxdb/pull/20797): Fix data race in TSM cache. Thanks @StoneYunZhao!
+1. [20811](https://github.com/influxdata/influxdb/pull/20811): Fix TSM WAL segment size computing. Thanks @StoneYunZhao!
 1. [20798](https://github.com/influxdata/influxdb/pull/20798): Deprecate misleading `retentionPeriodHrs` key in onboarding API.
 1. [20819](https://github.com/influxdata/influxdb/pull/20819): Fix Single Stat graphs with thresholds crashing on negative values.
 1. [20809](https://github.com/influxdata/influxdb/pull/20809): Fix InfluxDB port in Flux function UI examples. Thanks @sunjincheng121!
diff --git a/tsdb/engine/tsm1/wal.go b/tsdb/engine/tsm1/wal.go
index 77403819625..dfd6cf4abb9 100644
--- a/tsdb/engine/tsm1/wal.go
+++ b/tsdb/engine/tsm1/wal.go
@@ -242,6 +242,11 @@ func (l *WAL) Open() error {
 			}
 		}
 	}
+
+	if l.currentSegmentWriter != nil {
+		totalOldDiskSize -= int64(l.currentSegmentWriter.size)
+	}
+
 	atomic.StoreInt64(&l.stats.OldBytes, totalOldDiskSize)
 
 	l.closing = make(chan struct{})
@@ -379,6 +384,11 @@ func (l *WAL) Remove(files []string) error {
 
 		totalOldDiskSize += stat.Size()
 	}
+
+	if l.currentSegmentWriter != nil {
+		totalOldDiskSize -= int64(l.currentSegmentWriter.size)
+	}
+
 	atomic.StoreInt64(&l.stats.OldBytes, totalOldDiskSize)
 
 	return nil
@@ -565,7 +575,8 @@ func (l *WAL) newSegmentFile() error {
 		if err := l.currentSegmentWriter.close(); err != nil {
 			return err
 		}
-		atomic.StoreInt64(&l.stats.OldBytes, int64(l.currentSegmentWriter.size))
+
+		atomic.AddInt64(&l.stats.OldBytes, int64(l.currentSegmentWriter.size))
 	}
 
 	fileName := filepath.Join(l.path, fmt.Sprintf("%s%05d.%s", WALFilePrefix, l.currentSegmentID, WALFileExtension))
diff --git a/tsdb/engine/tsm1/wal_test.go b/tsdb/engine/tsm1/wal_test.go
index d846dc8346b..2a9bacc56b8 100644
--- a/tsdb/engine/tsm1/wal_test.go
+++ b/tsdb/engine/tsm1/wal_test.go
@@ -6,11 +6,13 @@ import (
 	"io/ioutil"
 	"os"
 	"reflect"
+	"sort"
 	"testing"
 
 	"github.com/golang/snappy"
 	"github.com/influxdata/influxdb/v2/pkg/slices"
 	"github.com/influxdata/influxdb/v2/tsdb/engine/tsm1"
+	"github.com/stretchr/testify/require"
 )
 
 func TestWALWriter_WriteMulti_Single(t *testing.T) {
@@ -701,6 +703,93 @@ func TestWALRollSegment(t *testing.T) {
 	}
 }
 
+func TestWAL_DiskSize(t *testing.T) {
+	test := func(w *tsm1.WAL, oldZero, curZero bool) {
+		// get disk size by reading file
+		files, err := ioutil.ReadDir(w.Path())
+		require.NoError(t, err)
+
+		sort.Slice(files, func(i, j int) bool {
+			return files[i].Name() < files[j].Name()
+		})
+
+		var old, cur int64
+		if len(files) > 0 {
+			cur = files[len(files)-1].Size()
+			for i := 0; i < len(files)-1; i++ {
+				old += files[i].Size()
+			}
+		}
+
+		// test zero size condition
+		require.False(t, oldZero && old > 0)
+		require.False(t, !oldZero && old == 0)
+		require.False(t, curZero && cur > 0)
+		require.False(t, !curZero && cur == 0)
+
+		// test method DiskSizeBytes
+		require.Equal(t, old+cur, w.DiskSizeBytes(), "total disk size")
+
+		// test Statistics
+		ss := w.Statistics(nil)
+		require.Equal(t, 1, len(ss))
+
+		m := ss[0].Values
+		require.NotNil(t, m)
+
+		require.Equal(t, m["oldSegmentsDiskBytes"].(int64), old, "old disk size")
+		require.Equal(t, m["currentSegmentDiskBytes"].(int64), cur, "current dist size")
+	}
+
+	dir := MustTempDir()
+	defer os.RemoveAll(dir)
+
+	w := tsm1.NewWAL(dir)
+
+	const segSize = 1024
+	w.SegmentSize = segSize
+
+	// open
+	require.NoError(t, w.Open())
+
+	test(w, true, true)
+
+	// write some values, the total size of these values does not exceed segSize(1024),
+	// so rollSegment will not be triggered
+	values := map[string][]tsm1.Value{
+		"cpu,host=A#!~#value": {tsm1.NewValue(1, 1.0)},
+		"cpu,host=B#!~#value": {tsm1.NewValue(1, 1.0)},
+		"cpu,host=C#!~#value": {tsm1.NewValue(1, 1.0)},
+	}
+
+	_, err := w.WriteMulti(values)
+	require.NoError(t, err)
+
+	test(w, true, false)
+
+	// write some values, the total size of these values exceeds segSize(1024),
+	// so rollSegment will be triggered
+	for i := 0; i < 100; i++ {
+		_, err := w.WriteMulti(values)
+		require.NoError(t, err)
+	}
+
+	test(w, false, false)
+
+	// reopen
+	require.NoError(t, w.Close())
+	require.NoError(t, w.Open())
+
+	test(w, false, false)
+
+	// remove
+	closedSegments, err := w.ClosedSegments()
+	require.NoError(t, err)
+	require.NoError(t, w.Remove(closedSegments))
+
+	test(w, true, false)
+}
+
 func TestWriteWALSegment_UnmarshalBinary_WriteWALCorrupt(t *testing.T) {
 	p1 := tsm1.NewValue(1, 1.1)
 	p2 := tsm1.NewValue(1, int64(1))