Abstract: Taking hydro pneumatic spring as elastic component, hydro pneumatic suspensions have built in non-linear stiffness which is harden in press stroke and soften in restore stroke. The close analytical functions of equivalent stiffness, dynamic equilibrium position with parameters of hydro pneumatic spring, road roughness and vehicle velocity were worked out base on 2DOF suspension model. The statistical linearization method was introduced and the nonlinear stiffness of hydro pneumatic spring was substituted with its Tyler series at the static equilibrium position. The numerical simulation is proposed to verify the results.