Abstract: The stability of karst slope is controlled by karst fracture. To study the instability and failure mechanism of karst mountain with deep and large fissures under the action of mining and fissure water, taking the case of “8·28” landslide in Pusa, Nayong County, Guizhou Province, as an example, based on field investigation, the characteristics of the landslide are analyzed. By using UDEC, the deformation and failure process and instability mechanism of karst mountain with deep and large fissures under the coupling of underground mining and fissure water seepage are investigated, and the mining-induced failure in the mountain with deep and large fissures is analyzed, in addition, the effect of fissure water seepage on the deformation of mining mountain is discussed. The results show that the influence of mining on the slope deformation is a process of continuous adjustment with mining. With the development of mining, the deformation of the slope changes from overall subsidence to extrusion in the middle of the slope. The deformation and failure of the slope are controlled by deep and large fissures and the fracture evolution form of the slope is determined by the longest fissure. When the slope is destroyed, the rock mass at the top of the slope downslides along main fracture, pushing the middle and lower slopes, forming a potential slip surface in the middle and lower slopes, and the potential sliding surface extends to the bottom of main fracture and penetrates through. The whole failure process of karst mountain body with deep and large fractures under the mining action is as follows: slope subsidence deformation-slope top toppling into the slope-main fissure closing-upper slope sliding along main fissure-the main fracture cracking-extruding in the middle of slope-the middle and lower rock mass being broken-potential sliding surface connecting-collapse forming. Before underground mining, the fissure water is the main factor causing slope deformation, but it has little effect on slope stability. After mining, the mining plays a leading role in slope deformation, and the contribution of hydraulic to slope deformation and failure depends on the mining stage. With the development of mining, the disturbance intensity of mining on the slope increases first and then decreases, and the effect of hydraulic pressure on slope deformation increases first and then decreases too. The stronger the mining disturbance is, the more significant the hydraulic effect is, conversely, the weaker the hydraulic effect is.