Abstract: Based on the comprehensive consideration of loading capacity, compact structure, light-weight and transmission efficiency of the tractor’s NGW planetary gear final transmission, the reliability optimization design method, setting on the knowledge of reliability theory and optimization technology and taking the stress and strength as random variables, was applied to build the multi-objective reliability optimization design model unlike the existing model. In the model, the mechanism’s smallest volume and highest efficiency were taken as the objective functions while reliability conditions of planetary transmission and gears’ fatigue strength, the planetary gear’s tooth configuration conditions and structural conditions were taken as the constraints, then the sequential quadratic programming algorithm in optimization toolbox of Matlab was used to solve the model. Compared with the original and common optimization design schemes, the final transmission volume reduces by 41.65% and 8%, respectively while its efficiency does not change, which shows that the method is practical, reliable and effective.