The invention discloses an air conditioning system and a control method thereof. The air conditioning system comprises a compressor, a reversing assembly, an outdoor heat exchanger, an indoor heat exchanger, a throttling device, a detection element and a controller; the compressor can work in a single-cylinder mode or a double-cylinder mode, and a first air suction port, an exhaust port and a second air suction port are formed in the compressor; the reversing assembly comprises a first valve port, a second valve port, a third valve port and a fourth valve port; the detection element is used for detecting the compression ratio of the compressor; and the controller is used for controlling the second air suction port to communicate with the first valve port when the compression ratio is smaller than the preset compression ratio so as to enable the compressor to work in the single-cylinder mode, and controlling the second air suction port to communicate with the fourth valve port when the compression ratio is larger than or equal to the preset compression ratio so as to enable the compressor to work in the double-cylinder mode. According to the air conditioning system and the control method thereof, the operation efficiency of the air conditioning system can be improved by detecting the compression ratio of the compressor to control the working mode of the compressor.