The invention discloses an air source heat pump defrosting system capable of switching two-stage compression into binary overlapping. The air source heat pump defrosting system comprises an indoor heat exchanger, an outdoor heat exchanger, an air-liquid separator, compressors, a high-pressure liquid reservoir, a four-way reversing valve, an economizer, stop valves, defrosting branches and a heat supplementary branch. The two ends of the first defrosting branch are connected with an inlet of the outdoor heat exchanger and an inlet of the air-liquid separator correspondingly. The two ends of the second defrosting branch are connected with an exhaust opening of the low-pressure compressor and an inlet of a cooling coiled pipe of the economizer correspondingly. One end of the heat supplementary branch is connected with the four-way reversing valve, and the other end of the heat supplementary branch directly leads into the high-pressure liquid reservoir. A heat supplementary solenoid valve is mounted on the heat supplementary branch. A heat supplementary sensor is arranged in a liquid phase of the high-pressure liquid reservoir. The procedure of a two-stage compression system is modified, heating and defrosting can run at the same time, the heating capacity is not attenuated, and the work conditions of the compressors are not changed; the impact on the system is low, and energy consumption is reduced; and on the premise of guaranteeing the indoor comfort degree, high efficiency and energy saving are achieved.