Liquid-air hammer hydraulic system of 40 tm using in a Shenyang factory was improved with large flow cartridge inserted valve integrated hydraulic system, and the backward process of this system was analyzed. According to the principle of the hydraulic system, topological structure of backward hydraulic system was established.By using the “grey-box” modeling theory, the mathematical modeling of hydraulic system was set up; and based on AMEsim software, the model of hydraulic system was constructed and the result from the simulation was got，which provides theoretical basis for the improving system and theoretical complement and guidance for future designs of forging equipment hydraulic system.