The disclosed operation method of a heat pump-type vehicle air conditioning system involves a step for setting a cold operating mode for raising the output air temperature of the air conditioner by means of a heater or a normal operating mode for raising the heating COP; a step for detecting the temperatures inside and outside the vehicle; and a step which, by selecting either the cold operating mode or the normal operating mode on the basis of the detected temperatures inside and outside of the vehicle, is for bypassing a condenser, controlling the opening of an electronic expansion valve contained in a bypass unit connected in series to a gas/liquid separating liquid coolant storage unit downstream of the aforementioned heater, and adjusting the pressure drop applied to the coolant.