Abstract: In order to estimate the sprung mass for the off-road vehicle in real time, an estimation method was proposed based on measuring the air pressure of the hydro-pneumatic air chamber and vehicle attitude. Based on the established seven-degree of freedom nonlinear full vehicle model with single tube hydro-pneumatic spring, the simulation was carried out by using the recursive least square estimation method with forgetting factor on the D-level road excitation. The influence of vehicle speed, filtering parameter and recursive least squares forgetting factor on the estimation performance was investigated. And the simulation results show that, the proposed method can provide the estimation with high accuracy and short response time.