The invention discloses an air conditioning system and a control method of the air conditioning system. The air conditioning system comprises a compressor, a reversing assembly, an outdoor heat exchanger, an indoor heat exchanger, a flash evaporator, a throttle device and a reversing valve. The compressor can work in a single-cylinder mode or a two-cylinder mode. A first air suction opening, a second air suction opening, an air supplementation opening, a control opening and an exhaust opening are formed in the compressor. The reversing assembly comprises a first valve opening, a second valve opening, a third valve opening and a fourth valve opening. An air outlet of the flash evaporator communicates with the air supplementation opening of the compressor. The reversing valve comprises a first opening, a second opening and a third opening. The first opening communicates with the control opening, the second opening communicates with the first valve opening, and the third opening communicates with the fourth valve opening. The first opening can selectively communicate with the second opening or the third opening, and therefore the control opening can selectively communicate with the first valve opening or the fourth valve opening. When the control opening communicates with the fourth valve opening, the compressor works in the single-cylinder mode. When the control opening communicates with the first valve opening, the compressor works in the two-cylinder mode.