diff --git a/src/type.ts b/src/type.ts
index 3679789..6e4e5a6 100644
--- a/src/type.ts
+++ b/src/type.ts
@@ -1563,7 +1563,7 @@ export class TypeResolver {
             }
         }
         if (!res && tsType.isUnion()) {
-            res = this.parseUnionType(tsType, typeNode as ts.UnionTypeNode);
+            res = this.parseUnionType(tsType, typeNode);
         }
         if (!res && this.isArray(tsType)) {
             if (!tsType.typeArguments) {
@@ -1669,11 +1669,15 @@ export class TypeResolver {
 
     private parseUnionType(
         tsUnionType: ts.UnionType,
-        unionTypeNode?: ts.UnionTypeNode,
+        unionTypeNode?: ts.Node,
     ): Type {
         const union_type = new TSUnion();
         /* 1. get type from typeNode firstly */
-        if (unionTypeNode && unionTypeNode.types) {
+        if (
+            unionTypeNode &&
+            ts.isUnionTypeNode(unionTypeNode) &&
+            unionTypeNode.types
+        ) {
             for (const typeNode of unionTypeNode.types) {
                 const type = this.generateNodeType(typeNode);
                 union_type.addType(type);
@@ -1683,6 +1687,17 @@ export class TypeResolver {
             if (!tsUnionType.types) {
                 return builtinTypes.get('any')!;
             }
+            if (
+                unionTypeNode &&
+                unionTypeNode.kind === ts.SyntaxKind.TypeReference
+            ) {
+                const tsTypeRawName = this.getTsTypeRawName(unionTypeNode)!;
+                if (builtinWasmTypes.has(tsTypeRawName)) {
+                    union_type.addType(builtinWasmTypes.get(tsTypeRawName)!);
+                    union_type.addType(builtinTypes.get('undefined')!);
+                    return union_type;
+                }
+            }
             for (const tsType of tsUnionType.types) {
                 union_type.addType(this.tsTypeToType(tsType));
             }
@@ -2912,15 +2927,6 @@ export class TypeResolver {
         }
     }
 
-    public static maybeBuiltinWasmType(node: ts.Node) {
-        const definedTypeName = (node as any).type?.typeName?.escapedText;
-        if (definedTypeName) {
-            if (builtinWasmTypes.has(definedTypeName)) {
-                return builtinWasmTypes.get(definedTypeName)!;
-            }
-        }
-    }
-
     public arrayTypeCheck(node: ts.Node): boolean {
         const parentNode = node.parent;
         if (
diff --git a/tests/samples/wasmType_in_otherType.ts b/tests/samples/wasmType_in_otherType.ts
index 628dc44..63bc99f 100644
--- a/tests/samples/wasmType_in_otherType.ts
+++ b/tests/samples/wasmType_in_otherType.ts
@@ -155,4 +155,19 @@ export function wasmTypeF32AsReturnType(): f32 {
 
 export function wasmTypeF64AsReturnType(): f64 {
     return 100.25;
-}
\ No newline at end of file
+}
+
+function wasmTypeInOptionalUnionType_inner(a_param?: i32) {
+    if (a_param) {
+        console.log(a_param);
+    } else {
+        console.log('undefined');
+    }
+}
+
+export function wasmTypeInOptionalUnionType() {
+    const a: i32 = 100;
+    wasmTypeInOptionalUnionType_inner(a);
+    const b = undefined;
+    wasmTypeInOptionalUnionType_inner(b);
+}
diff --git a/tools/validate/wamr/validation.json b/tools/validate/wamr/validation.json
index 5cb1cf7..a2325e9 100644
--- a/tools/validate/wamr/validation.json
+++ b/tools/validate/wamr/validation.json
@@ -4825,6 +4825,11 @@
                 "name": "wasmTypeF64AsReturnType",
                 "args": [],
                 "result": "100.25:f64"
+            },
+            {
+                "name": "wasmTypeInOptionalUnionType",
+                "args": [],
+                "result": "100\nundefined"
             }
         ]
     }