Abstract: Based on study of coal samples in mechanized longwall top-coal caving mining(MLTCM) face,anisotropy and inhomogeneity of top-coal fractures are quantitatively expressed,and inhomogeneity is applied in the study of permeability,the calculation results fits well with the observation results,which shows that multi-fractal method can provide a new method for the investigation of top-coal fracture and its permeability.