The interaction of small wheeled vehicles and soft sandy land will significantly affect the stability and security of the vehicle, and the research of the pressure-sinkage relationship is of great significance to the design and application of UGVs(Unmanned Ground Vehicles). In this paper, the finite element method is used to establish the pressure-sinkage model of the small wheeled vehicle and the soft sand, experiments are done by using the wheel-sand interaction testing system developed for UGVs.In considering the nonlinear deformation of the tire and sand, establishes the integrated radial static stiffness of the tire under 5 kinds of tire pressure, uses binary linear regression analysis to fit test data, determine the relationship of pressure-load-settlement. Aiming at the problem that the tire in the soft ground contact patch is difficult to be extracted , a reverse engineering technique is proposed to reconstruct the surface of the ground surface, which further verifies the accuracy of the coupled model. It provides a significant reference for improving performance of the UGVs and designing and manufacturing of new travelling mechanism.