diff --git a/expr/expr.go b/expr/expr.go
index bc0b57c3c..ee61b03bb 100644
--- a/expr/expr.go
+++ b/expr/expr.go
@@ -114,7 +114,7 @@ func (eval Evaluator) Fetch(ctx context.Context, exprs []parser.Expr, from, unti
 
 // Eval evaluates expressions.
 func (eval Evaluator) Eval(ctx context.Context, exp parser.Expr, from, until int64, values map[parser.MetricRequest][]*types.MetricData) (results []*types.MetricData, err error) {
-	rewritten, targets, err := eval.RewriteExpr(ctx, exp, from, until, values)
+	rewritten, targets, err := RewriteExpr(ctx, eval, exp, from, until, values)
 	if err != nil {
 		return nil, err
 	}
@@ -136,11 +136,19 @@ func (eval Evaluator) Eval(ctx context.Context, exp parser.Expr, from, until int
 		}
 		return results, nil
 	}
-	return eval.EvalExpr(ctx, exp, from, until, values)
+	return EvalExpr(ctx, eval, exp, from, until, values)
+}
+
+// NewEvaluator create evaluator with limiter and zipper
+func NewEvaluator(limiter limiter.SimpleLimiter, zipper zipper.CarbonZipper) (*Evaluator, error) {
+	if zipper == nil {
+		return nil, ErrZipperNotInit
+	}
+	return &Evaluator{limiter: limiter, zipper: zipper}, nil
 }
 
 // EvalExpr is the main expression Evaluator.
-func (eval Evaluator) EvalExpr(ctx context.Context, e parser.Expr, from, until int64, values map[parser.MetricRequest][]*types.MetricData) ([]*types.MetricData, error) {
+func EvalExpr(ctx context.Context, eval interfaces.Evaluator, e parser.Expr, from, until int64, values map[parser.MetricRequest][]*types.MetricData) ([]*types.MetricData, error) {
 	if e.IsName() {
 		return values[parser.MetricRequest{Metric: e.Target(), From: from, Until: until}], nil
 	} else if e.IsConst() {
@@ -198,7 +206,7 @@ func (eval Evaluator) EvalExpr(ctx context.Context, e parser.Expr, from, until i
 // applyByNode(foo*, 1, "%") -> (true, ["foo1", "foo2"], nil)
 // sumSeries(foo) -> (false, nil, nil)
 // Assumes that applyByNode only appears as the outermost function.
-func (eval Evaluator) RewriteExpr(ctx context.Context, e parser.Expr, from, until int64, values map[parser.MetricRequest][]*types.MetricData) (bool, []string, error) {
+func RewriteExpr(ctx context.Context, eval interfaces.Evaluator, e parser.Expr, from, until int64, values map[parser.MetricRequest][]*types.MetricData) (bool, []string, error) {
 	if e.IsFunc() {
 		metadata.FunctionMD.RLock()
 		f, ok := metadata.FunctionMD.RewriteFunctions[e.Target()]
@@ -210,14 +218,6 @@ func (eval Evaluator) RewriteExpr(ctx context.Context, e parser.Expr, from, unti
 	return false, nil, nil
 }
 
-// NewEvaluator create evaluator with limiter and zipper
-func NewEvaluator(limiter limiter.SimpleLimiter, zipper zipper.CarbonZipper) (*Evaluator, error) {
-	if zipper == nil {
-		return nil, ErrZipperNotInit
-	}
-	return &Evaluator{limiter: limiter, zipper: zipper}, nil
-}
-
 // FetchAndEvalExp fetch data and evaluates expressions
 func FetchAndEvalExp(ctx context.Context, eval interfaces.Evaluator, e parser.Expr, from, until int64, values map[parser.MetricRequest][]*types.MetricData) ([]*types.MetricData, merry.Error) {
 	targetValues, err := eval.Fetch(ctx, []parser.Expr{e}, from, until, values)
diff --git a/expr/expr_test.go b/expr/expr_test.go
index 3e660c7db..09b5acf88 100644
--- a/expr/expr_test.go
+++ b/expr/expr_test.go
@@ -207,7 +207,7 @@ func TestEvalExpr(t *testing.T) {
 
 			eval, err := NewEvaluator(nil, th.NewTestZipper(nil))
 			if err == nil {
-				_, err = eval.EvalExpr(context.Background(), exp, request.From, request.Until, metricMap)
+				_, err = EvalExpr(context.Background(), eval, exp, request.From, request.Until, metricMap)
 			}
 			if err != nil {
 				t.Errorf("error='%v'", err)
@@ -465,7 +465,7 @@ func TestRewriteExpr(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			eval, err := NewEvaluator(nil, th.NewTestZipper(nil))
 			if err == nil {
-				rewritten, newTargets, err := eval.RewriteExpr(context.Background(), tt.e, 0, 1, tt.m)
+				rewritten, newTargets, err := RewriteExpr(context.Background(), eval, tt.e, 0, 1, tt.m)
 
 				if err != nil {
 					t.Errorf("failed to rewrite %v: %+v", tt.name, err)
@@ -522,7 +522,7 @@ func TestEvalCustomFromUntil(t *testing.T) {
 			exp, _, _ := parser.ParseExpr(tt.target)
 			eval, err := NewEvaluator(nil, th.NewTestZipper(nil))
 			if err == nil {
-				g, err := eval.EvalExpr(context.Background(), exp, tt.from, tt.until, tt.m)
+				g, err := EvalExpr(context.Background(), eval, exp, tt.from, tt.until, tt.m)
 				if err != nil {
 					t.Errorf("failed to eval %v: %s", tt.name, err)
 					return