Abstract: To determine the development situation of surface cracks after the underground coal seam mining, this paper analyzed force of natural soil and its limit equilibrium state, and applied the Mohr-Coulomb criterion to study law of development regularity for surface cracks.Under action of the additional stress which generated by coal seam mining, this paper deduced calculation formula of critical horizontal deformation, crack depth and crack width. With assignment the test parameters of loess physical and mechanical properties in the mining area, and compared theoretical calculation formula results with the actual crack width, the results show that the theoretical formula calculation results in the article has a high credibility.