Taking the steep inclined coal seam group of Changgouyu coal mine for the engineering background, the paper analyzed the fracture development rule of overlying strata influenced by mining and seepage with discrete element method (DEM).Along with gradually minging, mining-induced fracture field order stack, the complex distribution of fracture evolvement in surrounding rocks is nonlinear increase trend, this trend will create the physical mechanism of water-blocking. Through calculating the fractal dimension of opening and closeing of fracture by fractal theory, the paper obtained that the relationship between crack closure rate and seepage flow of mined-out areas is non-symmetrical curve ,and the infiltration''s change fall behind the formation of water-blocking structure.