Abstract: In order to study the evolution laws of rock mass fracture at different depths of gob floor, firstly, the vertical and horizontal stress distribution at different locations of floor are discussed with the advancing of working face. During the advancement of the face, the vertical stress changes a lot compared with horizontal stress which changes a little, overall, the change process of floor stress can be regarded as: the confining pressure is constant, the axial pressure increases first and then decreases, and may reduce to zero or even change the axial pressure from compressive stress to tensile stress. Then taking the Xinhua Coal Mine as an example, the distribution laws of crack angle at different depths of floor and the relationship between floor depth and fracture number are acquired through UDEC simulation software, the results show that the number of fractures in the floor decreases with the depth of the floor until it is no longer developed. The cracks in the shallow depth are mainly with small angle or Ⅱtype shear crack, and the cracks in the large depth are mainly shear-expansion with lager angle.