A mathematic model of electro-hydraulic servo driving force control system for a heavy vehicle hydro-pneumatic suspension was established. Through analyzing the influence of the varying stiffness of hydro-pneumatic suspension to amplitude margin and phase margin, a relationship between stiffness of the load and controller parameters was obtained. In order to improve the dynamic performance of the hydro-pneumatic suspension testing system, a two-stage correction segment was introduced which can change its gain and its turning frequency automatically according to the stiffness of the load. Simulation and experimental results both indicate the validity of the proposed control method. 