Based on the analysis and study of the composite reducer, the allocation of transmission ratio is considered to be the key to optimal design of multi-level planetary gear reducer. In view of so many discussion and study of the minimum volume as the optimization objective existed, this paper regards the maximum load capacity as the optimal goal on condition that the center distance or size is given, and then establishes the optimal mathematical model. The related constraints are given according to the theoretical knowledge and practical design experience. In this paper, an optimal function procedure that based on genetic is used to optimize the mathematical model. The satisfied result has been worked out combined with the practical example. Compared to the original program, the load capacity of 17.8% is improved. In addition, some references are provided to the optimal design of planetary gear reducer.