A heat pump system for a vehicle, which makes refrigerant bypass an external heat exchanger and turns off a fan mounted on the external heat exchanger when temperature of the outdoor air is lower than setting temperature and the vehicle enters into an idle state in a heat pump mode, thereby continuously operating the heat pump mode even in the below zero temperature so as to keep heating performance, reducing consumption of electrical power without needing to operate an electric heater, and preventing excessive noise of a fan when the vehicle enters into an idle state in the below zero temperature.