Abstract: A new type of hydropneumatic suspension system was designed based on the traditional hydropneumatic suspension． The stiffness of the new suspension at the origin position can be adjusted，and theoretically the stiffness of suspension can be positive，zero or even negative． A matching design method and boundary for the suspension parameters at the origin were given for the new hydropneumatic suspension system when the suspension has zero stiffness at the origin position． The stability conditions of the new suspension system at the origin position was analyzed． The phase diagrams of the new suspension system under the free vibration conditions in balance position was obtained without damping，and the nonlinear relationship between the free vibration frequency and the amplitude of the suspension was obtained． This research enriches the design theory of hydro pneumatic suspension and provides a new design method for the design of hydropneumatic suspension．

Key words: mechanical engineering, hydropneumatic suspension, quasi-zero stiffness, energy method, free vibration