The geostress field and seepage field always exist in rock mass, the cavity excavation in the fractured rock mass will disturb the initial equilibrium and cause the change of geostress and seepage, so it is important to study the effect of geostress on the permeability of rock mass. The effect of rock block boundary size and initial geostress on the change of blowing discharge in the excavated cavity or tunnel is studied quantitatively by a numerical test. It is found that with the increase of block size and the ratio of initial horizontal stress to vertical stress, the blowing discharge in the excavated tunnel will be decreased. 