In order to study the frosting characteristics of air-source heat pumps and the heat transfer capacity degradation of evaporators under different relative humidity conditions, this paper constructs a dynamic frosting model of evaporators and a heat pump system model based on this model, and analyses the dynamic frosting characteristics of the evaporator and the performance degradation of the heat pump system under different relative humidity conditions. The results show that the higher the relative humidity, the faster the frosting rate, the more severe the evaporation pressure drop, the greater the increase rate of the pressure drop in the air side, and the faster the evaporator fin efficiency drop. Frosting of the evaporator will cause the performance degradation of the heat pump system. The greater the relative humidity of the air, the more serious the performance degradation of the system. Under the condition of the relative humidity of 90%, the system COP reduces by 2.69% when the frost forms for a duration of 30 minutes.