Disclosed therein are a heat pump system for a vehicle and a method of controlling the heat pump system, which variably controls only a compressor if the number of revolutions of the compressor is less than the upper limit of the number of the maximum revolutions of the compressor and operates an electric heater only when the number of revolutions of the compressor reaches the upper limit of the number of the maximum revolutions of the compressor in order to satisfy a target discharge temperature in a heat pump mode, thereby preventing that convergence of an air discharge temperature of the interior of the vehicle is deteriorated or becomes unstable when the compressor and the electric heater are variably controlled at the same time in order to satisfy the target discharge temperature.