Abstract: The performance of a small household air source heat pump water heater was tested in winter,and the performance changes of the unit were studied when the outdoor air temperature and water tank temperature were different. Based on the test data,the sensitive factors affecting the unit performance are analyzed,and the average energy efficiency in winter is analyzed according to the measured data in Tianjin. The results show that the lowest COP is 1.30,which has energy-saving effect,under the condition of ensuring water comfort and stable operation of the unit,45 ℃ is the best set temperature of the water tank. The outdoor air temperature offset is more sensitive to the system performance than the average temperature offset of the water tank. Under the test conditions,there is little difference in the average energy efficiency of the whole winter when the heat pump unit works alone and starts the electric auxiliary heating.If the user sets the unit to start heating at the highest temperature of the day,better energy-saving effect can be obtained.