In order to analyze the driving characteristics of tractorsemitrailer with the lateral degrees of freedom, a system dynamic model including body rolling and stiffness of steering system was established. Simulations were done under some typical operating conditions. Then, the traction saddle parameters were debugged and well extracted, and the system stability was investigated under the influence of these parameters, which provided reference for the designer. The changes of driving conditions and parameters of the traction saddle could cause body rolling and coupling angle and should be judged with the parameter sensitivity analysis. Simulation results have shown that the model can be applied to analyze vehicle characteristics appropriately.