In this paper, we make a numerical and simulant calculation of coupled stress and rock strength weakening caused by water seeping after reservoir impounding, using the theory of two-phase porous medium. The results show that due to water seeping, pore pressure in rock increases and coupled stress decraeses, so the effective stress in rock decreases and the shear strength should decrease. The decreasing increment is comparable to the value of the shear stress drop on seismic fault surface. Finally, some possible mechanisms about reservoir earthquake caused by water seeping are discussed.