A kind of hydro-pneumatic suspension with controllable stiffness and damping was designed. A complex nonlinear mathematic model of hydro-pneumatic suspensions was set up on the basis of comprehensive analysis of its working principle and structure. Simulation results comparison and theoretical analysis with the test data of bench test showed that the mathematic model is authentic. The volume of gas accumulator markedly affected the stiffness of hydro-pneumatic suspension, and the size of throttle notably affected the damping of the hydro-pneumatic suspension. The total volume of gas accumulator could be controlled by changing the number of gas accumulators, and the stiffness of hydro-pneumatic suspension could be controlled further more; the damping of hydro-pneumatic suspension could also be controlled by changing the size of throttle. 