Aim at the problem that traditional mechanical product design has a shortcoming for its long term of researching, and for its low effect, which made it hard to get to the optimal solution, Particle Swarm Optimization (PSO) is proposed in optimal design of a new-style circle guide rail which used in a rehabilitation robot's mechanical arm. The mechanical model was built based on purpose of the mechanical arm, and the important parameters were determined as design variables. At the same time, the boundary conditions of the design variables were determined by the strength check, the objective function of the minimum quality was established according to the optimal design objective. Reasonable structure parameters were obtained fastly and accurately by using PSO. The preference of optimal solution of a new-style mechanical arm is realized.