36.95%。The driving modes of a parallel-series hybrid electric vehicle (HEV) were analyzed. The system efficiency models for the HEV at the charging and discharging mode were proposed based upon the engine efficiency map, motor/generator efficiency map and battery efficiency map. The optimization of energy management strategy for the hybrid system was studied, whose optimization target is the maximum of discharging efficiency at the discharging mode, and the maximum of charging efficiency, the maximum of charging efficiency and charging power at the charging mode with the different state of charge of battery, and the optimal control torque and speed for engine, motor and generator were obtained under the different driving conditions. The simulation of fuel economy shows that the decreasing ratio of fuel consumption of the hybrid system reaches 36.95%.