With the mechanics model, the analytic formula of the deformation for the trapezium throttle slice was established. By the vehicle parameters and valve opening velocity, the formula of valve opening pressure was built. Based on this, through the relation of pressure with deformation and flux at valve opening, the design formula of throttle slice thickness and throttle-hole area were established, the influencing factors to the design parameters were analyzed. A practical example of design of hydro-pneumatic spring was given, and the performance test was conducted. The analysis and test results show that the design method is effective and the formulas for throttle slice thickness and throttle-hole area are accurate enough, and have important meaning for the hydro-pneumatic spring design.