In this study, a direct expansion solar-assisted heat pump system (DX-SAHP) with bare flat plate collectors is investigated experimentally in the enthalpy difference lab with a solar simulator and tested for space heating under different stable conditions. The influence of solar irradiation and outside temperature on the heating performance of DX-SAHP is analyzed. When outside temperature changes from 5℃ to 15 ℃ with solar irradiation of 300 W/m2，COP increases from 1.83 to 1.99, while the evaporator temperature and the condensing heat exchange rate increases by 8.9℃and 25%. Moreover, When solar irradiation changes from 100 W/m2 to 300 W/m2 with outside temperature of 15℃，COP of the system increases from 1.85 to 1.99, and the evaporator temperature and the condensing heat exchange rate increases by 2.0℃ and 21.0%, respectively. It is observed that when outside temperature is 7℃, without irradiation, after 30 min the evaporator is seriously frosted and the evaporating temperature is -0.77℃,while under irradiation of 80 W/m2 the evaporator is not frosted and the evaporating temperature and COP is respectively 3.12℃ and 15.5% higher than the former situation.