Reservoir induced seismicity is closely related with water permeating. The paper considers that the pre-earthquake activities in reservoir induced seismicity are mainly due to pore pressure diffusion produced by water permeating and the weakness of rock strength. As result of complexity of permeating property in underground rock beneath reservoir region, it is divided into two cases which are respectively homogeneous and inhomogeneous permeating in reservoir rock medium. Then, this paper has used the theory on pore pressure diffusion in porous media of solid-liquid two phases so as to calculate in numerical analog the pore pressure field produced by reservoir impounding. It is shown by calculated results that the distribution of pore pressure due to water permeating in inhomogeneous seeping models is corresponded well with the spacious position of reservoir seismicity. The time (1.8~45 days) of diffusion from peak of pore pressure to pre-earthquake focus of reservoir induced seismicity is about the same as the lag of the pre-earthquake activity produced by reservoir impounding.