Abstract: Slope angle of open pit mining is an important indicator of resource recovery rate, and steep slope mining can improve the mining slope angle, but it will aggravate the slope deformation, so it is necessary to control stress release and displacement deformation in steep slope mining. The relationship between in-situ stress and strength of soft rock is studied by Mohr-Coulomb criterion, and the criterion of stress redistribution and stress release is determined. The slope model of typical soft rock mine, the condition of groundwater and the mechanical index of rock and soil are selected. Through research, it is determined that the sliding mode of slope is “sitting-sliding” and slides along weak mudstone stratum. Before steep slope mining, the sliding stability of the slope along the roof and floor of the coal seam is 1.142 and 1.150, and the stability under the control of the floor is greater than that under the control of the roof. After steep slope mining, a large displacement and plastic deformation area is formed on the bench below 558 level, which is in the limit equilibrium state. The overall sliding stability along the coal seam floor is 1.033, which is consistent with the displacement failure mode of steep mining in numerical simulation.