A method for judging defrosting of heat pump water heater includes setting evaporation temperature sensor and defrosting temperature sensor on outdoor unit heat exchanger, detecting evaporation temperature T1 and defrosting temperature T2 during heating mode operation, setting evaporation temperature sensor and defrosting temperature sensor on outdoor unit heat exchanger, setting evaporation temperature sensor and defrosting temperature sensor on outdoor unit heat exchanger, setting evaporation temperature sensor and defrosting temperature sensor on outdoor unit heat exchanger, setting defrosting temperature sensor on outdoor unit heat exchanger and detecting defrosting temperature T2 during heating mode operation. Step S2: Judge T2-Whether T1 > [delta] T is tenable or not, if so, it is not frosted, and continues to operate under normal heating condition; 3 if not, proceed to step 3; 3, enter defrosting mode; Step S4: Judge T1 Whether T 2 >=[delta] T 'is true or not, if so, defrost mode should be maintained; If not, exit defrosting mode and return to heating mode. The method provided by the invention can judge whether the outdoor heat exchanger is frosted or not, so as to defrostthe outdoor heat exchanger accurately and efficiently.