Direct expansion solar-assisted heat pump system (DX-SAHP) is promising in energy saving applications, but the performance of DX-SAHP under frosting conditions is rarely reported in the published literatures. In this paper, a DX-SAHP system with bare solar collectors for space heating is designed and experimentally investigated in the enthalpy difference lab with a solar simulator. The system is tested under a range of frosting conditions, with the ambient temperatures from 7 degrees C to -3 degrees C, the relative humidities of 50%, 70% and 90% and the solar irradiances of 0 W/m(2), 100 W/m(2), 200 W/m(2) and 300 W/m(2). The conditions when the DX-SAHP system frosts are studied. Results show that solar irradiance as low as 100 W/m(2) can totally prevent frosting when the ambient temperature is above -3 degrees C and the relative humidity is 70%. Besides, the frosting process is observed to be slower than that of fin-and-tube heat exchangers. The evaporator is not seriously frosted and the system performance is not significantly influenced after 360 min of continuous operating. Moreover the effects of ambient parameters, including the ambient temperature and the relative humidity, especially solar irradiation, on the system performance are studied and analyzed. Solar irradiation can effectively prevent or retard frosting, and also improve the heating performance of the DX-SAHP system. The DX-SAHP system is proved to be applicable under frosting conditions. (C) 2016 Elsevier Ltd. All rights reserved.