An air conditioning system, comprising: a condenser (120, 130) for condensing a refrigerant; a first expansion device for throttling the refrigerant passed through the condenser; a phase separator arranged to separate the refrigerant passed through the first expansion device into a liquid refrigerant and a gaseous refrigerant; a second expansion device for throttling the liquid refrigerant separated by the phase separator; an evaporator (120, 130) for evaporating the refrigerant passed through the second expansion device; a compressor for introducing and compressing the refrigerant passed through the evaporator and the gaseous refrigerant separated by the phase separator; an injection valve for adjusting the amount of the gaseous refrigerant separated by the phase separator and injected into the compressor; and a control unit for controlling the first expansion device and second expansion device and the injection valve; characterized in that the control unit is arranged to decrease an opening degree of the first expansion device for a set period of time before and after the opening of the injection valve and in that when the air conditioning system is in a heating operation, upon opening of the injection valve in a start-up control step for the injection valve, the control unit is arranged to repeat a change process of changing an opening degree of the injection valve and a maintenance process of maintaining the opening degree until the injection valve is fully opened, wherein the control unit is arranged to decrease an opening degree of the first expansion device, maintain the decreased opening degree for a set period of time and then increase the opening degree of the first expansion device and maintain the increased opening degree until the injection valve is fully opened, and maintain an opening degree of the second expansion device. a condenser (120, 130) for condensing a refrigerant; a first expansion device for throttling the refrigerant passed through the condenser; a phase separator arranged to separate the refrigerant passed through the first expansion device into a liquid refrigerant and a gaseous refrigerant; a second expansion device for throttling the liquid refrigerant separated by the phase separator; an evaporator (120, 130) for evaporating the refrigerant passed through the second expansion device; a compressor for introducing and compressing the refrigerant passed through the evaporator and the gaseous refrigerant separated by the phase separator; an injection valve for adjusting the amount of the gaseous refrigerant separated by the phase separator and injected into the compressor; and a control unit for controlling the first expansion device and second expansion device and the injection valve; characterized in that the control unit is arranged to decrease an opening degree of the first expansion device for a set period of time before and after the opening of the injection valve and in that when the air conditioning system is in a heating operation, upon opening of the injection valve in a start-up control step for the injection valve, the control unit is arranged to repeat a change process of changing an opening degree of the injection valve and a maintenance process of maintaining the opening degree until the injection valve is fully opened, wherein the control unit is arranged to decrease an opening degree of the first expansion device, maintain the decreased opening degree for a set period of time and then increase the opening degree of the first expansion device and maintain the increased opening degree until the injection valve is fully opened, and maintain an opening degree of the second expansion device.