Abstract: Taking Longmenshan fault zone as the research background, basing on theory of strain energy of rock mass, this paper simulated energy source and energy release form of a simulated earthquake by using the software FLAC. Calculation results show that the total strain energy that can be released from the Longmenshan fault zone and its vicinity is approximately 3.24×10¹³ J under the action of horizontal stress increment 0.01 MPa. When stress triggering causes slip between fault planes, energy consumption for overcoming sliding friction on the surface is about 2.10×10¹³ J on fault plane. When the rock mass moves in the direction of gravity, energy consumption for overcoming gravity is 1.14×10¹³ J. Therefore it is deduced that, in a certain region, the energy released by stress triggering is approximately equal to the sum of consumption energy for overcoming the friction on fault surface and that for overcoming the gravity of regional rock mass. Strain energy may be released in a certain range, resulting in seismic effect. In the event of energy release of rock mass around fault triggered by stress increment, as for the the local region between the Yingxiu-Beichuan fault and Guanxian-Anxian fault in this paper, energy released centrally is about 7.67×10¹² J, equivalent to that for occurrence of an earthquake with M_S5.39.