A novel drive train system for pure electric vehicles is proposed. The system uses dual motor to achieve power-shift, and its transmission, which only uses one slide sleeve to shift gear uninterruptedly due to using one-way clutch, is based on parallel shaft gears and planetary gears. The structure and principle of the drive train is introduced. After the speed/torque relationship between power input and output is realized, downshift process can be described in detail. Accurate transmission model have been built and the dynamic control by PID controller of the downshift process is simulated. The simulation results show that the device has good performance in gear-shifting quality.