The invention belongs to the technical field of refrigeration equipment, and particularly relates to a defrosting control method for an air source heat pump. A defrosting system comprises a controller(1), a compressor (2), a four-way reversing valve (3), an evaporation temperature sensor (4), an evaporation heat exchanger (5), an environment temperature sensor (6), an electronic expansion valve (7), a condensation heat exchanger (8), a water temperature sensor (9) and a condensation temperature sensor (10). The environment temperature sensor (6) is used for monitoring the temperature of the environment. The evaporation temperature sensor (4) is used for monitoring the evaporation temperature. The water temperature sensor (9) is used for monitoring the water temperature. The condensation temperature sensor (10) is used for monitoring the condensation temperature. According to the method, a heat pump system can conduct defrosting at the best time during running under any condition, accordingly, the optimal heating efficiency can be achieved, an air source heat pump system runs more efficiently in a more energy-saving manner, and the advantage of energy saving of the air source heatpump technology is brought into full play.