In order to analyze the dynamic effects of elastic support on dynamic characteristics of marine planetary reducer, the dynamic model of transmission system is set up in consideration of the time-varying mesh stiffness and gear error, and the dynamic excitations of the system are calculated. The dynamic response model of the gearbox is set up by using finite element method and solved by using mode superposition method. Then the time history and spectrum of node acceleration of the gearbox are obtained, and the effect of support stiffness on vibration transfer and displacement of the gearbox are researched. The study provides useful theoretical guideline to the design of the marine gearbox.