Abstract: Heat transfer performance and heating efficiency of low temperature air-source heat pump water heaters (HPWT) (with quasi two stage compression and an intermediate economizer) were studied under different temperatures. Effects of temperatures on discharge temperature, back temperature, heating capacity, power consumption and coefficient of performance (COP) were investigated. The results show that heating capacity and COP of HPWT gradually reduce with the decrease of temperature. When the temperature changes from 20℃ to -15℃, the heat capacity and COP decrease by 64% and 54%, respectively. The heating time difference is over 3 times when heating 400 L cold water to 55℃. The COP value of HPWT is 1.80 when the outside temperature is -15℃, which is better than that of direct electric heating. These results may be applied for practical application of HPWT.