It has been widely discussed in geoscience community whether Coulomb stress change caused by Zipingpu reservoir loading activities has triggered the 2008 Wenchuan Ms8.0 earthquake.Many conclusions have come from 2D finite element simulations,which are quite different with the 3D results.In this paper,we quantitatively discussed the difference of the 2D and the 3D results.First,a comparison of the 3D finite element solutions and the analytic solutions was given to ensure the enough precision of the 3D finite element simulation with 1 million grid points.Then,the 2D and the 3D numerical solutions of the elastic effect and the seepage effect caused by Zipingpu reservoir were compared.The simulation results show that elastic load caused the thrust fault to be more secure,while the water pressure caused it to be more dangerous.The superimposed effect,however,didn't result an increase in the risk of Wenchuan earthquake.The comparison of the 2D and the 3D results show the 2D result exaggerated the truth.At the hypocenter of the Wenchuan earthquake,the computed stress of the 3D result is only 1/3 to 1/4 of the 2D result,thus in a serious study,3D modeling is necessary.