A complex nonlinear mathematic model of hydro-pneumatic suspension (HPS) was set up on the basis of comprehensive analysis of its working principle and structure in order to analyze the performance of HPS. Comparing results between the simulation results and the test data showed that the mathematic model is authentic. Moreover, the rigid property and damping capacity of HPS, as well as the effect of the main structural parameter were studied. The study results showed the HPS has a nonlinear rigid and a nonlinear damp performance.