The transmission system of warship’s gearbox is increasingly developing in the direction of high-speed, high-efficiency, heavy-load and light-weight, and the requirements of vibration are increasing as well. The gearbox designed according to static standard often has various problems of instability and vibration which greatly weaken the behaviors of gearbox and influence the invisibility performance of warship seriously. To improve the vibration characteristics of gearbox, vibration modal analysis of a warship’s gearbox is studied based on FEM. In order to avoid resonance in transmission, double-frequency dynamic optimization design of a gearbox is studied and discussed. The method and conclusions can be used as a reference of design of warship’s gearbox.