A 4 axle driven hill terrain vehicle was made in China. To improve the vehicle fatigue strength and reliability, the dynamics models of 4 axle-load were founded when it crossing obstacle. According to the dynamics models, the axleload distributions of the vehicle crossing obstacle were calculated when offroad adhesion coefficient φ is 0.1~1.0. The results showed its axleload distributions had more different among 4-axle when the vehicle crossing obstacle, the distribution law was analyzed. The design of 4-axle different strength and improving the 4-axle strength was put forward. The paper provided a mathematic model for reliability design and optimizing distribution of torque when the hill-ground crossing obstacle.