The characteristics of a hot water unit with direct-heated and frequency-variable air source heat pump were analyzed,including the theoretical coupled characteristics of water tank module and heat pump module,the operation characteristics of the heat pump units in variable environment conditions and the comparative influences of compressor frequency and condensation temperature.The results show that the compressor input power changes along with the ambient temperature in a parabolic curve,and reaches the highest at 20 ℃.Besides,the heat capacity is of linearly positive correlation with the ambient temperature and the outlet water flow has exponential relation to the ambient temperature.Moreover,when the effective volume coefficient and suction coefficient of the compressor keep constant,the refrigerant mass flow rate has linear and positive relationship with the ambient temperature.In addition,the heat COP is of exponential relation to the ambient temperature,and decreases with the outlet water temperature.It is concluded as well that the adjustment of compressor frequency can change heat capacity remarkably,and the decrease of condensing temperature can decrease the outlet water temperature(such as 42 ℃)in a certain degree,thus decreases the compressor input power and increases the outlet water flow.