To increase the driving range of electric vehicles in cold climate, air conditioning heat pump (ACHP) system is supposed to be the most effective solution. Working near 0 degrees C with high humidity, the microchannel outdoor heat exchanger (OHX) in system would experience badly frosting process, like traditional residential heat pump system. It would lead to a significant reduction of system performance without defrosting in time. In this article, experimental investigation has been implemented on the frosting process of ACHP system of electric vehicles which is with a microchannel OHX. The phenomenon of frosting distribution was observed, the frosted part on surface shows uneven with various flows paths. The typical frosting characteristics of an outdoor microchannel heat exchanger were also obtained. In a self-designed three-heat exchanger ACHP system, the inlet and outlet refrigerant temperature of OHX as well as the outlet air temperature of system decrease with increasing frosting coverage rate. The frosting phenomenon was analyzed with variation of ambient temperature and humidity. System influence by frosting was also studied with under different ambient conditions. When OHX begins to frost, the heating capacity reduction of system under different ambient conditions were both increased but the differences in the coefficient of performance (COP) variations under different ambient conditions were small as frosting progressed.