A method of controlling a heat pump system for a vehicle, wherein the heat pump system controlling method comprises: a first step of receiving various sensor values of the vehicle and calculating a target discharge temperature; a second step of determining whether the air-conditioning mode or the heat pump mode is selected according to the target discharge temperature or a passenger's selection; a third step of calculating a deviation between the target discharge temperature and an air discharge temperature of the interior of the vehicle, when the heat pump mode is selected as the determination result of the second step; a fourth step of calculating the number of revolutions of a compressor by the deviation of the third step; a fifth step of determining whether or not the number of revolutions of the compressor is less than upper limit of the number of the maximum revolutions of the compressor; a sixth step of variably controlling the compressor by the number of revolutions of the compressor according to the deviation calculated in the fourth step, if the number of revolutions of the compressor is less than the upper limit of the number of the maximum revolutions of the compressor as the determination result of the fifth step; a seventh step of determining whether or not the target discharge temperature is higher than the air discharge temperature of the interior of the vehicle, if the number of revolutions of the compressor is not less than the upper limit of the number of the maximum revolutions of the compressor as the determination result of the fifth step; and characterized in that it comprises an eighth step of controlling the operation of an electric heater 115 while keeping the operation of the compressor, if target discharge temperature is higher than the air discharge temperature of the interior of the vehicle as the determination result of the seventh step. a first step of receiving various sensor values of the vehicle and calculating a target discharge temperature; a second step of determining whether the air-conditioning mode or the heat pump mode is selected according to the target discharge temperature or a passenger's selection; a third step of calculating a deviation between the target discharge temperature and an air discharge temperature of the interior of the vehicle, when the heat pump mode is selected as the determination result of the second step; a fourth step of calculating the number of revolutions of a compressor by the deviation of the third step; a fifth step of determining whether or not the number of revolutions of the compressor is less than upper limit of the number of the maximum revolutions of the compressor; a sixth step of variably controlling the compressor by the number of revolutions of the compressor according to the deviation calculated in the fourth step, if the number of revolutions of the compressor is less than the upper limit of the number of the maximum revolutions of the compressor as the determination result of the fifth step; a seventh step of determining whether or not the target discharge temperature is higher than the air discharge temperature of the interior of the vehicle, if the number of revolutions of the compressor is not less than the upper limit of the number of the maximum revolutions of the compressor as the determination result of the fifth step; and characterized in that it comprises an eighth step of controlling the operation of an electric heater 115 while keeping the operation of the compressor, if target discharge temperature is higher than the air discharge temperature of the interior of the vehicle as the determination result of the seventh step.