The servo control hydraulic system of connecting rod expansion and break was designed. The system simulation model of the process to be expanded and broken was built based on AMESim.The piston displacement curve that expansion and break hydraulic cylinder reaches the preset position to be expanded was obtained by simulation,the influence of the proportional coefficient on the piston displacement of the expanding and breaking hydraulic cylinder was analyzed,and the optimal proportional coefficient was determined to be 5 000.Aimed to the requirements of different expansion and break forces,the characteristics of system expansion and break were simulated, and the influences of expansion and break forces on the displacement, the velocity, the pressure and the flow of the upper and lower cavities of the cylinder were studied. The results show that the designed servo control hydraulic system of connecting rod expansion and break can adapt to the expansion and break conditions of different expansion and break forces, which provides reference for further optimizing the hydraulic system of connecting rod expansion and break.