Power trains are an important component of the tidal current energy conversion systems; however, the variable drive-torque and unbalanced moments produced by flow shear and turbulence cause power trains to vibrate. When the scale of the tidal current turbine increases, the vibration problem becomes prominent. The dynamic characteristics of power trains are complex. In this study, the power train of a 650 kW horizontal-axis tidal current turbine was studied. The power train adopted a low-speed-ratio semi-direct drive scheme proposed by Zhejiang University. A mathematical model of the power train was constructed. The influence of external excitations (including tidal current condition, unbalanced moments, and internal excitation) on dynamic characteristics was studied using simulations and sea trials. The simulation results showed that the radial and torsional vibrations of the low- and the high-speed shafts increased when the tidal current velocity increased. The fluctuation range of the bearing load increased under increases in the pitch and yaw moments. The meshing motion of the planetary gear plays a leading role in the vibration of low- and high-speed shafts. The power train model was validated by comparing the results of the experiment and simulation.