A great majority of current researches on obstacle negotiation focus on the critical state where CG position get over the obstacle using kinematics model. However, the models assume that the sinkage of the robot can be ignored and do not take into account whether the terrain can provide enough drawbar pull to keep the robot moving. While, when robots running on the soft terrain, the sinkage of the robot is very big that it can be ignore and it is very easy for the robots to slip. In order to solve that problem, this paper focuses on the effects of terrain physical parameters in the process of obstacle negotiation. Based on this point, the interaction models of wheel-terrain and track-terrain are built according to terramechanics. Using these models, two key postures of climbing obstacles are discussed, and the relationship between maximum surmountable obstacle-height and geometry parameters of the robot are obtained. Then maximum draw pull tests of wheel-track model and wheels model are designed to compare experimental results with theoretical results. Finally, experiments on surmounting obstacle performance of tracked robot are carried out, and the results show that our model is more exact than the CG kinematic model.