A direct-expansion solar-assisted heat pump (DX-SAHP) water heater is studied numerically. A simulation model, which includes the models of components and the frost formation of the collector/evaporator, is established to research the heating performance of the system and the growth of frost. The calculation results conform well to the experiment, which verifies the accurate predictions. On the basis of the model, the influences of the meteorological parameters on the system performance under frosting conditions are analyzed. It is found the formation of frost can be effectively inhibited by decreasing the ambient temperature below a specific value. What's more, the collector efficiency can also be improved effectively during the incipient growth of frost crystals. Besides, higher solar radiation intensity can significantly improve the heating performance and reduce the frosting rate. By contrast, the enhancement in wind speed will have the same effect, but only slightly. The system coefficient of performance (COP) reaches above 2.75 with the reliable heating performance under typical frosting conditions, and the collector/evaporator presents the superiority in delaying frosting in comparison with the traditional evaporator for air source heat pump.