Abstract: We discuss the model of action and mechanism of special water-resisting key strata, a barrier layer in pressure relief gas extraction, and propose the typical structure for barrier layer. The results show that the barrier layer is a special form of water-resisting key strata. According to the relative position of barrier layer and protected coal seam and their position in the “three zones” of overburden rock, four stratigraphic structures of the barrier layer are divided. Further, the gas flow pattern is obtained, and there are four kinds of mechanisms of barrier layer to control the gas flow pattern. They are the compression of displacement and deformation of rock strata under the action of superimposed stress, the expansion and block effect of weak lithologic and minerals, the filling function of fluid carrying material, the fracture intensity.