The design optimization of two-stage cylindrical helical gear reducer is discussed in this paper. The volume and the center gear distance are adopted as objective functions separately, and the particle swarm optimization(PSO) method is used to improve the design quality. The design variables contain discrete variables, which are converted to discrete variables by the nearby principle in order to improve the running efficiency. Inertia weight coefficient is used in the PSO algorithm to adjust the coverage speed. The optimization result shows that our method is better than other optimization method, and it proves that PSO method is meaningful to the reducer’s design.