The Planetary gear train is widely used in large heavy-duty machinery such as helicopter and ship due to its large transmission ratio, high transmission efficiency, strong bearing capacity and small space occupied. In this paper, the dynamic characteristics of a two-stage planetary gear reducer equipped on a marine are studied. Firstly, according to the characteristics such as periodic time-varying meshing stiffness of gear meshing process, the time-varying tooth surface friction and friction moment are also considered as periodic functions and taken into the dynamic equation. Therefore, the formulas of friction force and friction moment are derived. Then, by combining the Euler beam element method with the concentrated mass method, and a dynamic model of the double-helical planetary gear system is established. Finally, the Newmark method was used to solve the dynamic equation and the displacement in the direction of vibration was obtained. Meanwhile, the FFT method was used to analyze the main frequency factors affecting the vibration, and the analysis results are helpful to reduce the vibration of the planetary gear system.