The unexpected Wenchuan earthquake occurred on the Longmen Shan Fault, which is not active recently. It will exert an important influence on the society and scientific community. Especially, the tectonic mechanisms of the Wenchuan earthquake will be paid more and more attention. In the paper, the distribution of the stress field and its evolution with time in the eastern Qinghai-Tibetan plateau is simulated by means of viscoelastic finite element method, with gravity being considered in the model. The result shows that the stresses are transferred to and concentrated in the Longmen Shan area from a state of decentralization in space. Based upon the results from previous studies and the present paper, the dynamical process of the Wenchuan earthquake is assumed as: with the movement of eastward mass flow of the Qinghai-Tibet Plateau, some part of material would accumulate on the west brim of the Sichuan basin since the stable and strong Sichuan basin blocks the eastward flow, resulting in the rise of the Longmenshan Mountains. The value of the normal stresses on the fault plane which is steep (the dip angle of 50°~70°）and of listric shape, increases with the eastward motion of the plateau of the western Sichuan, making the fault plane lock more tightly. At the same time, the accumulated material will exert stronger and stronger forces to the hanging wall of the Longmen Shan fault, therefore the shear stresses on the fault are becoming greater and greater with the movement of eastern mass flow of the Tibet. Once the shear stresses exceed the friction strength, the fault will be unlocked, and it will slip suddenly. Therefore an earthquake occurs. Also, the simulation result suggests that the average recurrence time of earthquakes in the Longmen Shan area is about 3163 years, which is in agreement with the result by other methods.