In this paper, the 3D Finite Element Method was combined with a Discontinuous Deformation Analysis (DDA + FEM) to study the influence of the Wenchuan earthquake on the stability of the tectonic blocks system of the Qinghai–Tibet Plateau under the background of northward extrusion of the Indian Plate and obstruction of the strong crust of the Sichuan basin. With constraints from GPS data and focal mechanisms, we first calculated the velocity and stress fields of the region. Then, we numerically simulated the rupture process of the seismogenic fault of the 2008 Wenchuan earthquake in overthrust form with a right-lateral component. We then studied the movement and deformation of the tectonic blocks that was caused by the earthquake and its influence on the stress state of the boundary faults. The numerical simulation indicates that the large earthquake causes the most tectonic blocks in the study zone at approximately a 1-mm deviation toward the northeast; in particular, the Wenchuan seismogenic fault obviously deviated eastwards, and the maximum deviation reaches 1.5 m. It seems that the large earthquake causes the Bayankala block at the west side of the Longmenshan fracture zone to extend farther eastward. The Wenchuan earthquake causes the Coulomb failure stresses on the boundary faults of the tectonic blocks in the study zone to change to different extents. The maximum change of the Coulomb failure stress reaches about 0.2 MPa at both ends of the Wenchuan seismogenic fault, the south section of the Xianshuihe fracture zone and part of the southeast section of the Dongkunlun fracture zone in the upper crust. Additionally, the Coulomb failure stresses also increase to different extents between 0.01 and .02 MPa in regions such as the southernmost end of the Xianshuihe fracture zone, part of the southeast section of the Dongkunlun fracture zone, and so on.