The invention discloses a heat accumulating defrosting system for an air source heat pump water heater. The system comprises a compressor, a gas-liquid separator, a four-way reversing valve, an outdoor heat exchanger, a capillary, a plate-type phase change heat accumulator, a one-way electromagnetic valve, a dry filter, a first stop valve, a shower room, a second stop valve, an external water tank heat exchanger and a water storage tank. The defrosting process comprises that: condensed waste heat of the heat pump water heater system and wastewater waste heat of the shower room are reclaimed through the phase change heat accumulator, and the flowing direction of a refrigerant in the plate-type phase change heat accumulator and the outdoor heat exchanger is changed by steering of the four-way reversing valve, so that switching of functions of waste heat energy accumulation and energy release defrosting of the system is realized. The system has simple structure and is convenient to control and easy to maintain and the like.