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Based on the requirement of CF700 tractor cab suspension parameters, the elastic force and damping force model of hydro-pneumatic spring was established using fluid mechanics theory. The key parameters of hydro-pneumatic spring were calculated, and the damping adjustable hydro-pneumatic spring was designed. The effect of the excitation, throttle valve size and check valve size on the output force were studied by experiments. Test result is shown that the damping force can be in a large range of adjustment. The throttle valve size had influence on output force of both compression and rebound stroke, while the check valve size just had influence on the output force of compression stroke, and the throttle valve size had more obvious influence on the output force. The effectiveness of design ideas and methods were verified, and the research base of tractor cab hydro-pneumatic suspension was provided.