Southwest mountainous area is an important energy base in China. There are a large number of mining engineering, which is one of the important inducements of many landslide disasters. Under the action of mining, the development of mountain fracture zone controls the stability of mountain. The development height of the fracture zone was quantitatively calculated by a simple beam model based on key layer theory, and the results were verified by UDEC numerical simulation. The results show that under the influence of coal mining, the overburden loses its support and flexure deformation occurs. The cracks on roadway top gradually expand to the surface. The cracks have the characteristics of large scale and difficult to heal, and they finally form a permanent fracture zone. Considering the location of the cut hole, the nature of the rock stratum, the mining range and other factors, the subsidence value of the mountain rock stratum under the mining action is quantified, and a theoretical formula is proposed to effectively predict the height of the fracture zone. Based on UDEC numerical simulation test, the prediction of fracture zone height is verified. The result is close to the theoretical calculation result, and the maximum error is 7%. It is feasible to use the model to predict the overestimation of fracture zone.