In order to solve the gear shifting power interruption problem in electric vehicles with automatic mechanical transmission, this paper proposes a novel dual-motor powertrain system including two motors and one gearbox, which can realize nine combined gears. Based on the working state of the power source, the clutch and the synchronizer in each working mode, as well as the operating characteristics of the vehicle, the dynamics of each gear shifting stage is analyzed, and the dynamic equations are established. The shifting process is simulated based on the co-simulation platform of MATLAB and AMEsim. Finally, a hardware in the loop test system including solenoid valve is designed and developed, and the gear shifting process from 01 to 11 is selected as a representative scheme for the experimental validation. The experimental results show that compared with the control strategy that the motor target speed is greater than the speed of the driven plate of the clutch, the control strategy that the motor target speed is lower than the speed of the driven plate of the clutch proposed in this paper reduces the shift time and sliding friction work by 37.4% and 87.3% respectively.