In this paper, 3-D Finite Element Method combined with Discontinous Deformation Analysis (DDA+FEM) is used with the constraints of 3-D tectonic blocks of Qinghai-Xizang Plateau and neighboring areas to study the pregnant mechanism of Wenchuan earthquake. The topography on both sides of Longmenshan fracture zone, the marked change in thickness of the crust and its layers, and the influence of a stronger crust in the Sichuan Basin and Ordos block on the eastern side of the fracture zone are taken into consideration. With the GPS data constrained on the boundary, the calculated velocity and stress fields agree with the observed GPS data and the focal mechanism distribution results. On this basis, the distribution of risk factors on the boundary faults of tectonic blocks, which characterize overall influences of shear stress and normal stress on fault face, etc, is calculated numerically. The results show that the sections with high risk factors coincide with the areas where most of the M_S>7 earthquakes have occurred in recent decades, including M_S=8.0 Wenchuan earthquake in 2008. Trial computation of situations of two different structures on both east and west side of Longmenshan fracture zone show that significant changes of the topography on both sides, the thickness and the layers of the crust, and the physical properties played important roles in the pregnant mechanism of the Wenchuan earthquake. The calculated distribution maps of strain intensity show that the eastern edges of Qinghai-Xizang Plateau are close to the strain intensity steep change zone. Of the zones with steep change in strain intensity in eastern edge of the plateau, Longmenshan fracture zone shows the steepest change. The intensity contour on the east side of the fracture zone decay faster than the west side. In addition, the calculated strain energy density contour shows that the width and the strike direction of the upper and middle layers of the Longmenshan fracture zone is located in the same direction of the high strain energy density band. In the upper layer of the crust, low strain energy density is found on both sides. In the middle layer of the crust, the strain energy density decreases from west to east: the western side has high strain energy density, the eastern side has low strain energy density. Our results show that under strong subduction of Indian Plate, constraints of 3-D tectonic blocks in Qinghai-Xizang Plateau and influence of special tectonic environment on the west and east side of the Longmenshan fracture zone, the seismic fault area, where Wenchuan earthquake occured, had accumulated sufficiant strain energy. It indicates that the area is in mechanical instable state.