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Through the analysis of Running cycle of Changsha City, and according to the automobile running equation, the minimal energy of driving and braking under the sampling points of 0.1 second interval is analyzed and summed, the recycle of braking energy of motor is analyzed, two formulas for calculating the energy consumption of BEV are derived based on the running and braking cycle, the expression of energy consumption per unit distance is also derived. The optimization objective function for powertrain ratio are established with the energy consumption per unit distance and the dynamic factor under the Changsha loop running cycle and the main road running cycle. In order to assure the dynamic and economy performance of the designed car, the ratio boundary constraints is established with the effect of maximum speed and peak torque of the motor and the lower state of charge of the battery. And the powertrain ratio optimization is presented with a particle swarm optimization based on the simulated annealing, the weighting coefficient and the method of dealing with Constraints based on penalty function are introduced. And the results of optimization indicate that the dynamic factor is increased and the energy consumption per unit distance of the designed car is reduced under a group of weighting coefficient.