diff --git a/.chronus/changes/fix_one_variant_union-2024-6-9-14-12-37.md b/.chronus/changes/fix_one_variant_union-2024-6-9-14-12-37.md
new file mode 100644
index 0000000000..2f6a54278d
--- /dev/null
+++ b/.chronus/changes/fix_one_variant_union-2024-6-9-14-12-37.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+fix wrong handling for one variant union
\ No newline at end of file
diff --git a/packages/typespec-client-generator-core/src/types.ts b/packages/typespec-client-generator-core/src/types.ts
index f30295d154..e07de8adf2 100644
--- a/packages/typespec-client-generator-core/src/types.ts
+++ b/packages/typespec-client-generator-core/src/types.ts
@@ -354,7 +354,8 @@ export function getSdkUnionWithDiagnostics(
     return diagnostics.wrap(diagnostics.pipe(getAnyType(context, type)));
   }
 
-  if (nonNullOptions.length === 1) {
+  // if a union is `type | null`, then we will return a nullable wrapper type of the type
+  if (nonNullOptions.length === 1 && nullOption !== undefined) {
     retval = diagnostics.pipe(getClientTypeWithDiagnostics(context, nonNullOptions[0], operation));
   } else if (
     // judge if the union can be converted to enum
diff --git a/packages/typespec-client-generator-core/test/types/union-types.test.ts b/packages/typespec-client-generator-core/test/types/union-types.test.ts
index e55230d393..aec84baa02 100644
--- a/packages/typespec-client-generator-core/test/types/union-types.test.ts
+++ b/packages/typespec-client-generator-core/test/types/union-types.test.ts
@@ -584,4 +584,26 @@ describe("typespec-client-generator-core: union types", () => {
     strictEqual(unionAsEnumInternal.usage, UsageFlags.Input | UsageFlags.Output);
     strictEqual(unionAsEnumInternal.access, "internal");
   });
+
+  it("union with only one literal", async function () {
+    await runner.compileWithBuiltInService(
+      `
+        @usage(Usage.input | Usage.output)
+        @access(Access.public)
+        model Test {
+          name: TestUnion;
+        }
+
+        union TestUnion {
+          "A"
+        }
+      `
+    );
+    const sdkType = getSdkTypeHelper(runner);
+    strictEqual(sdkType.kind, "enum");
+    strictEqual(sdkType.name, "TestUnion");
+    const values = sdkType.values;
+    strictEqual(values.length, 1);
+    strictEqual(values[0].value, "A");
+  });
 });