An air conditioning system can improve the cooling/heating performance of the system because the injection of refrigerant into the compressor is achieved. The air conditioning system can further improve the cooling/heating performance in a low temperature region by injecting refrigerant as a two-phase refrigerant or a superheated vapor state into the compressor. The air conditioning system can prevent damage of the compressor and further improve reliability by controlling such that the ratio of a liquid refrigerant in the refrigerant injected into the compressor may be less than a set value.