In order to improve the efficiency of the power system in electric vehicles, a mode switching control strategy of electric vehicles assisted with electric machine is presented.First, a dual-motor-type electric vehicle was obtained,which was amounted with a low-power motor on the the original structure of a traditional electric vehicle; Secondly, the dynamic analysis and the energy-balance analysis of its power system were completed, and then its efficiency at each mode was represented; Then, to make the efficiency of the power system maximum ,a mode switching control strategy was formulated, which determined the target mode target of the vehicle and target torque of the two motors; Finally, in Advisor simulation platform CYC_NEDC cycle condition, the mode switching control strategy was simulated, and simulation results showed that, compared with conventional electric cars, this control strategy made the working efficiency of the power system increased by about 6%.