Abstract: Simulation program of a direct expansion solar assisted heat pump water heater system was coded based on the distributed parameter and homogeneous flow mathematical models of solar collector/evaporator and condenser, the lumped parameter models of compressor and thermostatic expansion valve and the refrigerant charge model. The calculation and the experiment results were compared. Based on the calculation results, the operational parameters and the performances of the system were obtained. The effects of changing the refrigerant charge on heating time, the superheat of collector outlet, evaporating pressure, condensing pressure, power of the compressor, collector efficiency and coefficient of performance (COP) were analyzed. Some references to judge optimum refrigerant charge were proposed.