The complex structure of underwater vehicle drive system, coupled with the prominent dynamic issues resulting from multi-source dynamic excitation, which affects the long endurance and low noise performance of the vehicle. In response to the above issues, the electromechanical-fluid coupling dynamic model for motor-planetary gear train-propeller-flow field is developed by lumped-parameter method. The model accounts for nonlinear factors such as the voltage drop across the inverter switches and the dead-time of the permanent magnet synchronous motor, which cause output torque ripple. The open water performance curves of the propeller are obtained by using the Reynolds-averaged Navier-Stokes(RANS) equations, which also takes into account factors such as hydrodynamic damping and dynamic viscosity. Under the premise of satisfying the hydrodynamic performance, the optimal propulsion parameters are selected, and the blades torque is calculated as the system load according to the corresponding advance coefficient. Numerical methods are employed to solve the dynamic response of the planetary gear transmission system and the variation in its dynamic performance is analyzed under input-output load fluctuations and parameter excitation. This study reveals the dynamic characteristics and parameter influence laws of planetary gear system in electromechanical-fluid coupling system, providing a theoretical basis for the dynamic design and optimal matching of underwater vehicle drive system.