The heat storage device is proposed to avoid the frequent on-off state under partial load and larger variation of the supply & return water temperature and longer defrosting time of air-to-water heat pump with plate heat exchanger for heating and cooling. The heat storage device is composed of a water tank and the inner double pipe heat exchanger, appendages and electric heater. The energy storage can reduce on-off times of the compressor. Energy storage and the electric heater make up the shortage of heating during defrosting. Performance test shows that on-off times of the compressor under partial load are reduced, defrosting time is shorted, and more steady running parameters are achieved. The designed heat storage can improve the unit performance and should be suitable for new designed of air-to-water heat pump.