Abstract: Taking the evaporator with defrosting device as the research object,a dynamic model of air source heat pump（ASHP）based on the ideal minimum amount of supplemental heat required for defrosting and frost volume dimensionless correlation is established,and its accuracy is validated based on experimental data,the effects of different environmental factors on the performance of the evaporator is analyzed by numerical simulation. The results show that the heat transfer coefficient of the evaporator of the rear-mounted and crossmounted system is greater than that of the front-mounted and traditional electric auxiliary heating system,and the thickness of the frost layer is smaller and more evenly distributed. When the relative humidity is certain,the heat transfer coefficient of the evaporator rises with the temperature. While the temperature is unchanged,this coefficient reduces with the decline of relative humidity,and the frost layer is more uniform. The influence of temperature to heat transfer coefficient of the evaporator is greater than that of relative humidity.