Abstract: The geological conditions of the existing coal seam group in Faer Coal Mine in Guizhou province are taken as the engineering background, and the law of slope instability caused by coal seam mining is studied by analyzing the deformation law of the mountain, mining subsidence law of slope surface and change law of slope angle in the mining process. Mining under the condition of mountain occurrence is easy to cause mountain deformation and failure, increase of slope angle, etc, which leads to the local shear failure surface of the slope and causes the slope instability. With the analysis of mining slope instability under mountain condition, the safety analysis and evaluation of mountain slope can be conducted, and prevention measures can be taken for local shear failure to ensure safe mining under mountain condition.