Abstract: In this paper,a mathematical model for the direct-expansion solar assisted heat pump water heater system is developed with verified experimental data. On this basis,the influence of design parameters of collector/evaporator on system performance are analyzed by sensitivity analysis method. Meantime,the sensitivity coefficient is introduced as an evaluation index to determine the key design parameters of collector/evaporator. The results show that the area of the collector/evaporator has the most significant effect on the heating coefficient of performance and heating capacity,and the sensitivity coefficients for them are 0.201 and 0.223,respectively. Finally,the area and absorption rate of collector/evaporator are determined as key design parameters.