To reduce the gearbox volume and the impact of running while improving bearing capacity, pin and pin hole profile parameters and material parameters were quoted to establish the mathematical model of normal contact force on the basis of Hertz contact theory. Five reducer dimensions were selected as the design variables to construct eight nonlinear constraint equations, and a multi-objective optimization model of optimizing contact force and reducer volume was established. Evaluation function method and penalty function method were used to verify the optimization model. A comparison of the results before and after the optimization showed that the reducer volume and the normal contact force obtained an optimization rate of 13.1% and 24% respectively. ADAMS was applied for the dynamic simulation of the normal contact force, and the simulation diagrams of time domain and frequency domain were obtained. The results showed that its average optimization rate reached 29.4% and coincided with the theoretical results.