A shock absorber test instrument is designed in order to test the comprehensive stiffness of the helical spring and the hydro-pneumatic spring of the hydro-pneumatic suspension of engineering vehicles, and to analyze the main factors that influence the body amplitude frequency characteristics. Firstly, one quarter of the hydro-pneumatic spring nonlinear vibration model is established according to the mass concentration principle and Newton′s second law. Then, the system equation is transformed into nonlinear algebraic equations by the Taylor expansion and the harmonic balance method, and the system steady-state solution is obtained. The results show that the solution is close to the one obtained using the Runge-kutta method. On this basis, the main influence of the amplitude-frequency characteristics of the car body is analyzed. The results show that increasing initial nitrogen pressure, road excitation amplitude, and mass ratio has little effect on body resonance frequencies, but serves to increase body amplitude. At the same time, the tire stiffness increases both the body amplitude and the time of the body to steady-state vibration.