The invention provides an air conditioner defrosting system. The system comprises a gas-liquid separator, a compressor unit and an oil separator which are connected in series with two ports of a four-way valve, wherein an air stop valve, an indoor unit, a liquid stop valve, a high-pressure liquid storage device, a third electromagnetic valve, an outdoor electronic expansion valve and an outdoor heat exchanger are connected in series with the other two ports of the four-way valve; a first pipeline of a heat storage radiator is connected to a pipeline for connecting the high-pressure liquid storage device with the third electromagnetic valve and a pipeline for connecting the outdoor heat exchanger with the four-way valve; a second pipeline of the heat storage radiator is connected to a pipeline for connecting the gas-liquid separator with the four-way valve and a pipeline for connecting the outdoor electronic expansion valve with the third electromagnetic valve; and a third pipeline of the heat storage radiator is connected to a pipeline for connecting the four-way valve with the air stop valve. The invention also provides an air conditioner defrosting method. The system can generate heat while operating in the defrosting operation process; and heat generated in the heating process is not required to be consumed in the defrosting operation process, and the defrosting operation process is performed by heat in a phase transition heat exchanger, so that a heating process and a defrosting process can be performed at the same time, and the performance of an air conditioner is improved.