An air-conditioning system comprising: a phase separator for separating a flowing refrigerant into a gas-phase refrigerant and a liquid-phase refrigerant; an evaporator for evaporating the liquid-phase refrigerant separated by the phase separator; a compressor having a first compression part into which the refrigerant having passed through the evaporator is introduced and a second compression part into which both the gas-phase refrigerant separated by the phase separator and the refrigerant having passed through the first compression part are introduced; a control unit (c) for simultaneously or selectively controlling the amount of the refrigerant introduced into the phase separator and the amount of the refrigerant discharged from the phase separator; a first expansion valve for controlling the amount of the refrigerant introduced into the phase separator; and a second expansion valve for controlling the amount of the liquid-phase refrigerant discharged from the phase separator and introduced into the evaporator; characterised in that the air-conditioning system further comprises: a refrigerant control valve for controlling the amount of the gas-phase refrigerant discharged from the phase separator and introduced into the second compression part, wherein the control unit controls all the valves to prevent the introduction of the liquid-phase refrigerant into the compressor; wherein the control unit closes all the valves to prevent the introduction of the liquid-phase refrigerant into the compressor when the operation of the air-conditioning system is initiated; and wherein the control unit controls the opening amount of the refrigerant control valve when the refrigeration cycles is stabilized. a phase separator for separating a flowing refrigerant into a gas-phase refrigerant and a liquid-phase refrigerant; an evaporator for evaporating the liquid-phase refrigerant separated by the phase separator; a compressor having a first compression part into which the refrigerant having passed through the evaporator is introduced and a second compression part into which both the gas-phase refrigerant separated by the phase separator and the refrigerant having passed through the first compression part are introduced; a control unit (c) for simultaneously or selectively controlling the amount of the refrigerant introduced into the phase separator and the amount of the refrigerant discharged from the phase separator; a first expansion valve for controlling the amount of the refrigerant introduced into the phase separator; and a second expansion valve for controlling the amount of the liquid-phase refrigerant discharged from the phase separator and introduced into the evaporator; characterised in that the air-conditioning system further comprises: a refrigerant control valve for controlling the amount of the gas-phase refrigerant discharged from the phase separator and introduced into the second compression part, wherein the control unit controls all the valves to prevent the introduction of the liquid-phase refrigerant into the compressor; wherein the control unit closes all the valves to prevent the introduction of the liquid-phase refrigerant into the compressor when the operation of the air-conditioning system is initiated; and wherein the control unit controls the opening amount of the refrigerant control valve when the refrigeration cycles is stabilized. a refrigerant control valve for controlling the amount of the gas-phase refrigerant discharged from the phase separator and introduced into the second compression part, wherein the control unit controls all the valves to prevent the introduction of the liquid-phase refrigerant into the compressor; wherein the control unit closes all the valves to prevent the introduction of the liquid-phase refrigerant into the compressor when the operation of the air-conditioning system is initiated; and wherein the control unit controls the opening amount of the refrigerant control valve when the refrigeration cycles is stabilized.