The heat storage device is proposed to avoid the frequent on-off state under partial load, large variation of the supply and return water temperature and long defrosting time of air-to-water heat pump with plate heat exchanger for heating. The heat storage device is composed of a water tank, the inner double-pipe heat exchanger, appendages and the electric heater. The energy storage can reduce on-off times of the compressor. The energy storage and the electric heater can make up the shortage of heating during defrosting. Performance test shows that on-off times for the heat storage device unit compressor under partial load is about four times per hour compared six times per hour for the plate heat exchange unit. Defrosting time is reduced by 78 s by heat storage device unit and 84 s by heat storage device with electric heater unit. More steady running parameters, inlet heating water temperature of heat storage device or heat storage device with electric heater unit is about 35°C, while that of plate heat exchanger reduces to 30.73°C. Therefore, the designed heat storage device can improve the unit performance and is suitable for the new projects with air-to-water heat pumps.