Abstract: Numerical model for coal seam group mining in Datong Mining Area was built by using the software FLAC^3D, the fractures development during mining process in coal seam group were numerically studied, the development laws of the failures with thickness and face length of 3-5^# coal seam was analyzed. The results show that, with the increase of coal seam mining, the increase of fracture development trend declined; the height of fracture increases with the increasing of thickness of 3-5^# coal seam; when the coal thickness is greater than 13 m, the fracture height increases quickly; the height of fracture zone increases the face length. When the face length is larger than 190 m, the increase of fracture development trend declined; when the coal thickness is 15 m, the working surface length shortened to 180 m, which can make sure that the fracture zone of 3-5^# coal seam and the goaf of 15^# coal seam are not related.