diff --git a/pkg/runtime/function/cast_decimal.go b/pkg/runtime/function/cast_decimal.go
new file mode 100644
index 00000000..8296c23f
--- /dev/null
+++ b/pkg/runtime/function/cast_decimal.go
@@ -0,0 +1,101 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package function
+
+import (
+	"context"
+	"fmt"
+)
+
+import (
+	"github.com/pkg/errors"
+)
+
+import (
+	"github.com/arana-db/arana/pkg/proto"
+)
+
+// FuncCastDecimal is https://dev.mysql.com/doc/refman/5.6/en/cast-functions.html#function_cast
+const FuncCastDecimal = "CAST_DECIMAL"
+
+var (
+	_defaultDecimalPrecision proto.Value = proto.NewValueInt64(10)
+	_defaultDecimalScale     proto.Value = proto.NewValueInt64(0)
+)
+
+var _ proto.Func = (*castDecimalFunc)(nil)
+
+func init() {
+	proto.RegisterFunc(FuncCastDecimal, castDecimalFunc{})
+}
+
+type castDecimalFunc struct{}
+
+func (a castDecimalFunc) Apply(ctx context.Context, inputs ...proto.Valuer) (proto.Value, error) {
+	if len(inputs) != 3 {
+		return nil, errors.New("The Decimal function must accept three parameters\n")
+	}
+
+	val, err := inputs[0].Value(ctx)
+	if err != nil {
+		return nil, errors.WithStack(err)
+	}
+	d, err := val.Decimal()
+	if err != nil {
+		return proto.NewValueFloat64(0), nil
+	}
+
+	precision, err := inputs[1].Value(ctx)
+	if err != nil {
+		return nil, errors.WithStack(err)
+	}
+
+	if precision == nil {
+		precision = _defaultDecimalPrecision
+	}
+
+	p, err := precision.Int64()
+	if err != nil {
+		return nil, errors.WithStack(err)
+	}
+
+	scale, err := inputs[2].Value(ctx)
+	if err != nil {
+		return nil, errors.WithStack(err)
+	}
+
+	if scale == nil {
+		scale = _defaultDecimalScale
+	}
+	s, err := scale.Int64()
+	if err != nil {
+		return nil, errors.WithStack(err)
+	}
+
+	// M must be >= D
+	if p < s {
+		return nil, errors.WithStack(fmt.Errorf("for float(M,D), double(M,D) or decimal(M,D), M must be >= D"))
+	}
+
+	return proto.NewValueString(d.StringFixed(int32(s))), nil
+
+}
+
+func (a castDecimalFunc) NumInput() int {
+	return 3
+}
diff --git a/pkg/runtime/function/cast_decimal_test.go b/pkg/runtime/function/cast_decimal_test.go
new file mode 100644
index 00000000..28ef42b6
--- /dev/null
+++ b/pkg/runtime/function/cast_decimal_test.go
@@ -0,0 +1,65 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package function
+
+import (
+	"context"
+	"fmt"
+	"testing"
+)
+
+import (
+	"github.com/stretchr/testify/assert"
+)
+
+import (
+	"github.com/arana-db/arana/pkg/proto"
+)
+
+func TestCastDecimal(t *testing.T) {
+	fn := proto.MustGetFunc(FuncCastDecimal)
+	assert.Equal(t, 3, fn.NumInput())
+
+	type tt struct {
+		inFirst  proto.Value
+		inSecond proto.Value
+		intThird proto.Value
+		out      string
+	}
+
+	for _, it := range []tt{
+		{proto.NewValueInt64(15), proto.NewValueInt64(4), proto.NewValueInt64(2), "15.00"},
+		{proto.NewValueInt64(15), proto.NewValueInt64(4), proto.NewValueInt64(0), "15"},
+		{proto.NewValueInt64(15), proto.NewValueInt64(4), (nil), ("15")},
+		{proto.NewValueInt64(15), nil, nil, ("15")},
+		{proto.NewValueInt64(15), proto.NewValueInt64(0), nil, ("15")},
+		{proto.NewValueFloat64(8 / 5.0), proto.NewValueInt64(11), proto.NewValueInt64(4), "1.6000"},
+		{proto.NewValueString(".885"), proto.NewValueInt64(11), proto.NewValueInt64(3), "0.885"},
+		{proto.NewValueString(".885"), proto.NewValueInt64(11), proto.NewValueInt64(4), "0.8850"},
+		{proto.NewValueString(".885"), proto.NewValueInt64(2), proto.NewValueInt64(1), "0.9"},
+		{proto.NewValueString(".885"), proto.NewValueInt64(2), nil, "1"},
+		{proto.NewValueString(".885"), proto.NewValueInt64(2), proto.NewValueInt64(0), "1"},
+		{proto.NewValueString(".885"), proto.NewValueInt64(20), proto.NewValueInt64(0), "1"},
+	} {
+		t.Run(it.out, func(t *testing.T) {
+			out, err := fn.Apply(context.Background(), proto.ToValuer(it.inFirst), proto.ToValuer(it.inSecond), proto.ToValuer(it.intThird))
+			assert.NoError(t, err)
+			assert.Equal(t, it.out, fmt.Sprint(out))
+		})
+	}
+}