An air conditioning system and a control method thereof. The air conditioning system includes a main circuit and a first subcooling circuit, wherein the main circuit has: a main compressor and an injector; a gas cooler and a gas-liquid separator connected between the main compressor and the injector; and a main throttling element and an evaporator connected between the gas-liquid separator and the injector; and wherein the first subcooling circuit has: a first subcooling compressor, a first condenser, a first subcooling throttling element and a first subcooler connected in sequence; wherein the first subcooler is further disposed in a flow path between the outlet of the injector and the gas-liquid separator.