The ambient air is commonly used as low-temperature heat source for heat pump operation. However, the coefficient of performance(COP) of the air to water heat pump is decreased with the ambient air temperature drop. In this study to solve this problem, the AVACTHE(Automatic Variable Area Capillary Type Heat Exchanger) with 3 levels of heat exchange area(0, 1,495.4, 1,794.5$ extrm{cm}^2$)was installed in the refrigerant circuit of the heat pump. The AVACTHE effect on the performance of heat pump was tested with the ambient air temperature variation. The COP improvement of the heat pump could be achieved by the AVACTHE installation when below -5$^{circ}C$ of the ambient air temperature.