diff --git a/ddl/ddl_api.go b/ddl/ddl_api.go
index 6cbbb92805872..9245f3fbb8ef6 100644
--- a/ddl/ddl_api.go
+++ b/ddl/ddl_api.go
@@ -6041,6 +6041,10 @@ func checkAlterTableCharset(tblInfo *model.TableInfo, dbInfo *model.DBInfo, toCh
 		return doNothing, nil
 	}
 
+	if err = checkIndexLengthWithNewCharset(tblInfo, toCharset, toCollate); err != nil {
+		return doNothing, err
+	}
+
 	for _, col := range tblInfo.Columns {
 		if col.GetType() == mysql.TypeVarchar {
 			if err = types.IsVarcharTooBigFieldLength(col.GetFlen(), col.Name.O, toCharset); err != nil {
@@ -6064,6 +6068,30 @@ func checkAlterTableCharset(tblInfo *model.TableInfo, dbInfo *model.DBInfo, toCh
 	return doNothing, nil
 }
 
+func checkIndexLengthWithNewCharset(tblInfo *model.TableInfo, toCharset, toCollate string) error {
+	// Copy all columns and replace the charset and collate.
+	columns := make([]*model.ColumnInfo, 0, len(tblInfo.Columns))
+	for _, col := range tblInfo.Columns {
+		newCol := col.Clone()
+		if field_types.HasCharset(&newCol.FieldType) {
+			newCol.SetCharset(toCharset)
+			newCol.SetCollate(toCollate)
+		} else {
+			newCol.SetCharset(charset.CharsetBin)
+			newCol.SetCollate(charset.CharsetBin)
+		}
+		columns = append(columns, newCol)
+	}
+
+	for _, indexInfo := range tblInfo.Indices {
+		err := checkIndexPrefixLength(columns, indexInfo.Columns)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 // RenameIndex renames an index.
 // In TiDB, indexes are case-insensitive (so index 'a' and 'A" are considered the same index),
 // but index names are case-sensitive (we can rename index 'a' to 'A')
diff --git a/ddl/ddl_api_test.go b/ddl/ddl_api_test.go
index 3ae596031a9a6..b3a6e425f48b1 100644
--- a/ddl/ddl_api_test.go
+++ b/ddl/ddl_api_test.go
@@ -267,3 +267,15 @@ func TestCreateDropCreateTable(t *testing.T) {
 	require.Less(t, create0TS, dropTS, "first create should finish before drop")
 	require.Less(t, dropTS, create1TS, "second create should finish after drop")
 }
+
+func TestFix56930(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t; create table posts (id int auto_increment primary key, title varchar(500) character set utf8, subtitle varchar(500) character set utf8, unique key(title, subtitle));")
+	tk.MustGetErrMsg("alter table posts convert to character set utf8mb4;", "[ddl:1071]Specified key was too long (4000 bytes); max key length is 3072 bytes")
+	tk.MustExec("drop table if exists t; create table t(a varchar(1000) character set utf8, primary key(a));")
+	tk.MustGetErrMsg("alter table t convert to character set utf8mb4;", "[ddl:1071]Specified key was too long (4000 bytes); max key length is 3072 bytes")
+	tk.MustExec("drop table if exists t; create table t(a varchar(1000) character set utf8, key(a));")
+	tk.MustGetErrMsg("alter table t convert to character set utf8mb4;", "[ddl:1071]Specified key was too long (4000 bytes); max key length is 3072 bytes")
+}