Abstract: Based on fractal evolution characteristics of coal fracture, the effect of fracture development on gas desorption is discussed. With the coal mass simplified as fracture-matrix structure, the relationship between fracture length and matrix block size is analyzed. Laws of coal matrix block size evolution and gas desorption change are obtained based on fractal characteristics of coal fracture development. Gas desorption rate increases linearly at initial stage of fracture evolution, and increases rapidly with decreasing of coal matrix block size by abundant microfractures at later stage. Due to the changes of fracture fractal dimention determined before and after mining, it is found that matrix block size could be decreased by 4/5 and gas desoption rate could be increased by more than 21.6 times. The fracture development caused by mining will have a great effect on gas diffusion behavior.