After analyzing the national standard GB/T23137-2008 and GB/T 21362-2008 for the cyclic and the static heating types of heat pump water heaters, it is found in the stipulated measurement method of heating capacity some kinds of errors may be introduced and the judgement of performance may be influenced.Here another testing method — drainage method was proposed.In the method, the test can start, only when the temperature in the tank reaches 55 ℃.Then hot water was discharged from outlet, meanwhile 15 ℃ water was poured in from inlet.Until the temperature difference between discharged water and inlet water is equal to or less than 0.5 ℃, the experiment will be over.The drainage curve graph was drawn, and the heating capacity of the units was calculated.The results show that comparing with the standard method, the heating capacity and COP by the drainage method both increase by 10.2% for the cyclic heating type, and increase by 12% for the static heating type.The study shows that the drainage method has higher measurement accuracy in the heat pump water heater performance test, and will have a promising application.