Abstract: Owing to the periodic cyclic load in the engine crankshaft, the crank pin will produce excessive Mises stress and further cause fatigue cracks. In order to decrease the Mises stress and improve the fatigue life of the crank pin, the crank pin stepped fillet will be designed and optimized in this article. Firstly, the flexible mulit-body dynamics simulation on the crank connecting rod is conducted in order to obtain the load spectrum of the crank pin. Then, the optimization model taking the minimum stress of the crank pin as the optimization objective and the value of R₁、R₂ and H as the design variables is built based on RSM (response surface methodology); and the optimization model is solved. It is concluded through comparative analysis on the optimization model that the stepped fillet can effectively reduce the Mises stress on the crank pin, and can reduce the possibility of premature fatigue failure due to excessive stress at the fillet. It has certain reference value to the structure design of crankshaft fillet.