800MN giant forging hydraulic press is a strategical equipment which is related to national security, economic development, and the base equipment for aerospace manufacturing industry. The synchronous control system is the essential device to the giant forging hydraulic press, whose synchronization control performance quality is good or bad will directly determine the quality of the product. Synchronous balance cylinder is the key part of the synchronous control system. Based on the mechanics model, the synchronous balance cylinder was designed. With the help of the software MARC，the finite element analysis was carried on, the structural weaknesses were optimized, which effective reduce the torsion of the piston and the degree of the bending stress of the piston and significantly decrease the equivalent stress of the axis and piston . The results provide the reference for the design of similar products.