An air conditioning system, comprising: a condenser for condensing a refrigerant; a first expansion device for throttling the refrigerant passed through the condenser; a phase separator for separating the refrigerant passed through the first expansion device into a gaseous refrigerant and a liquid refrigerant; an injection valve for controlling an amount of the gaseous refrigerant passed through the phase separator; a second expansion device for throttling the liquid refrigerant passed through the phase separator; an evaporator for evaporating the refrigerant passed through the second expansion device; a compressor for compressing the refrigerant passed through the evaporator and the refrigerant passed through the injection valve; and a control unit for detecting a value of at least one operating parameter and determining a target opening degree of the first expansion device on the basis of a stored set value corresponding to the detected value of the operating parameter, characterized in that the control unit is arranged to detect a current opening degree of the first expansion device and the opening amount of the first expansion device is changed a plurality of times by a change (B1, B2 and B3; or C1, C2 and C3) in opening degree until the current opening degree of the first expansion device reaches the target opening degree, wherein the change (B1, B2 and B3; or C1, C2 and C3) in opening degree is determined in consideration of the current opening degree of the first expansion device such that the closer the opening degree reaches to the target opening degree, the smaller change (B1, B2 and B3; or C1, C2 and C3) in opening degree becomes. a condenser for condensing a refrigerant; a first expansion device for throttling the refrigerant passed through the condenser; a phase separator for separating the refrigerant passed through the first expansion device into a gaseous refrigerant and a liquid refrigerant; an injection valve for controlling an amount of the gaseous refrigerant passed through the phase separator; a second expansion device for throttling the liquid refrigerant passed through the phase separator; an evaporator for evaporating the refrigerant passed through the second expansion device; a compressor for compressing the refrigerant passed through the evaporator and the refrigerant passed through the injection valve; and a control unit for detecting a value of at least one operating parameter and determining a target opening degree of the first expansion device on the basis of a stored set value corresponding to the detected value of the operating parameter, characterized in that the control unit is arranged to detect a current opening degree of the first expansion device and the opening amount of the first expansion device is changed a plurality of times by a change (B1, B2 and B3; or C1, C2 and C3) in opening degree until the current opening degree of the first expansion device reaches the target opening degree, wherein the change (B1, B2 and B3; or C1, C2 and C3) in opening degree is determined in consideration of the current opening degree of the first expansion device such that the closer the opening degree reaches to the target opening degree, the smaller change (B1, B2 and B3; or C1, C2 and C3) in opening degree becomes.