Abstract: For large inertia servo system, the influence of structure stiffness on stability is great and it cannot be ignored. Taking the hydraulic vibration servo system of mould for example, the mathematical model of the mould hydraulic vibration servo system was established and the general conditions for system stability were obtained. Considering the influence of structure stiffness, the mathematic model of hydraulic cylinder to load system was also established, and the effect of structure stiffness on the stability was studied through calculation and analysis. The results showed that the comprehensive resonant frequency was the main factor for limiting hydraulic servo system for large inertia servo system. In this case, it must improve the structure stiffness with improving hydraulic inherent frequency. And when considering the influence of joint stiffness, the stability of closed-loop system was better than the half closed-loop system. The research can provide references for analysis of the stability of large inertia servo system.