Abstract: The mining slope in mountains area of southwestern China has some common characteristics. They include high and steep terrain,"hard stratum over soft stratum" slope structures, gently inclined strata, well-developed joints with high dip angle, and strong mining activities beneath the slope. The slope has not obvious movement basin. The slope cracks caused by underground mining are large and corresponding to the mined-out area. It has a complicated formation mechanism. This article takes the case of Jieniangping deformable rock mass in Guizhou province. It analyses the genetic mechanism of slope cracks using numerical simulation. Result shows that slope occurs cracks along joints and all the way to surface. Due to the impact of high and steep terrain and underground mining, the cracks continues to expand with repeated mining. The slope has a toppling failure trend toward surface. It proposes that slope deformation process includes three stages: mining disturbance, the tension at the crest, deformation increase through numerical simulation. Due to repeated mining in multi-coal strata, slope deformation has experienced four stages, namely initial deformation stage, constant deformation stage and accelerated deformation stage and stable deformation.