Abstract: To investigate the mechanism of mutual influence between tractor and semi-trailer during driving process, vehicle dynamic model with 11 degrees of freedom were established with the aid of vehicle force analysis. Matlab was used to obtain the numerical simulation results for tractor semi-trailer. To testify the correctness of vehicle dynamic model and numerical simulation, real vehicle test were conducted. Research results show that: while vehicle drives on road with high adhesion coefficient, lateral acceleration and yaw rate for tractor and semi-trailer are quite close to each other, and trailer's motion response are hysteretic compared to tractor; vehicle is apt to encounter with rollovers but sideslip and jackknife; and when vehicle rolls over, trailer's axles lift first, then tractor's rear axles, tractor's front axles are the latest to lift.