Abstract: In order to avoid the continuous decline in battery state of charge（SOC）, reduce the regulation frequency of engine speed and generator torque, and improve the vehicle fuel economy when the range extender electric vehicle is working in range-extended mode, an auxiliary power unit（APU） power flow optimization method was proposed in this paper. Firstly, the measured vehicle real-time electric power was revised according to the generator efficiency, battery SOC and power variation rated in turn, and then the target APU power was obtained. Meanwhile the engine on/off control strategy was designed based on the vehicle power demand and battery SOC. Then, the target engine speed and generator torque was obtained according to APU fuel-electricity conversion efficiency features and optimal APU fuel-electricity conversion efficiency curve. Finally, bench experiment was performed under new European driving cycle（NEDC）, federal test procedure（FTP） and high way fuel economy test（HWFET） driving cycles. The results showed that the proposed approach avoided the continuous decline in battery SOC, and improved the vehicle fuel economy effectively, and it also lowered the performance demands on engine speed and generator torque control systems.