This paper describes the optimal design of the reduction gearbox of a tillage machine. The minimum center diameter was selected as the objective, and the contact fatigue strength, bending fatigue strength, condition of nonintervention, and oil film thickness ratio of the gearbox were applied as constraint conditions. The optimal model was solved by a Matlab program. The results show that the center diameter of the reduction gearbox decreased by about 10%. The resulting decrease in weight and volume led to a reduction in the amount of gearbox material and a consequent decrease in production cost.