In current years, searching for substitute energy such as the wind and ocean power of renewable energy is an important subject due to the petroleum shortage. The gear box is the key mechanism in the system of wind and ocean power. Moreover, the main function of the system is to generate electricity by speeding up the rotative velocity. However, the power of environment changes momentarily and makes the turbine a variable input. In order to improve the efficiency of generating electricity, it should get the constant input to keep the high quality of generating electricity. First of all, the study focuses on the gear box and synthesizes a constant speed mechanism of planetary gear train which degrees of freedom is two. In the mechanism, inputs are the variable speed turbine and a constant speed motor. When they input the mechanism, a constant speed output will be made to supply the generator. Secondly, it is necessary to set up the real object of the constant speed mechanism of planetary gear train. Finally, observing the real condition from the experiment to prove the theory is correct.