A failure diagnosis method of a coolant temperature sensor for a vehicle, may include determining, by a controller, whether or not it is in a flow stop state that stagnates the flow of coolant by a flow control valve; obtaining, by the controller, an engine outlet coolant model temperature when it is in the flow stop state; and diagnosing, by the controller, as a failure of an engine outlet-side outlet water temperature sensor when the coolant model temperature is equal to or greater than a reference temperature and the flow stop state is maintained by the flow control valve.