The invention belongs to the field of air conditioning systems, particularly relates to a double-cylinder variable-capacity compressor air conditioning system and a control method thereof. The air conditioning system comprises a large-cylinder circulation loop and a small-cylinder signal loop, wherein the large-cylinder circulation loop is formed by connecting a variable-capacity compressor, a first three-way pipe, a four-way selector valve, an outdoor heat exchanger, a throttle mechanism, an indoor heat exchanger and a second three-way pipe in sequence, and the small-cylinder signal loop is formed by connecting the variable-capacity compressor, the first three-way pipe and the three-way selector valve in sequence. In the control method of the double-cylinder variable-capacity compressor air conditioning system, the opening and the closing of the small-cylinder signal loop are controlled by the switch on/off condition of the three-way selector valve, thereby realizing reliable and active control for a small cylinder of an air conditioner, ensuring the operation of the air conditioner and bringing about good comfort to users; in addition, frost on the outdoor heat exchanger can be removed faster and more completely in the automatic defrosting process, and therefore, users feel more comfortable.