With the improvement of energy saving and environmental protection requirements, the market share of heat pump water heater (HPWH) is gradually increasing. In order to increase the heat storage capacity and improve the performance of the system, an air-source HPWH with storage tank using water and finned phase change material (PCM) for heat storage is designed and experimentally studied. Performances of the HPWH for the first operation when the PCM is in heat storage and the second operation when the PCM is in heat discharge were compared and discussed. Furthermore, performances between the HPWH using water-PCM for heat storage and conventional HPWH with standard water tank were compared. The results show that the volume of the storage tank has little increase, but the heat storage is increased by 14%. Besides, when water is heated from 15 to 55 degrees C, the first operation time for the HPWH is almost the same as the normal operation time of the conventional HPWH, but the second operation time is shortened by 13%. The results also show that the HPWH has good water temperature uniformity and its COP was improved.