Abstract: K117 landslide of Yong-Ning highway is a cutting slope with two excavation stages in argillaceous siltstone of monoclinic layer. The formation mechanism of this landslide is complex because it is the long run-out plane sliding as far as 35m and the sliding pit as long as 25m on slide bed when excavated. Based on the detailed investigation to its basic characteristics and development process, the landslide is simulated by joint finite element method in the process of its slide of three stages and landslide treatment. Firstly, the paper reappears the whole gestating process and stability evolving regulations of the landslide. It analyzes the plane sliding mechanism and the physical and mechanical characteristics of sliding surface. Next, it discusses the long run-out sliding mechanism induced by excavating relaxation and filling water within fissure. Lastly, it proposes six suggestions to predict and control this landslide. The results shows that, when the weak structure planes in monoclinic layer is exposed on free surface after excavating, it can activate unloading relaxation of the whole slope and cause the great imbalance thrust of landslide. Meanwhile, landslide is very sensitive because of rainfall infiltration in stretched cracks. Due to change of potential energy to kinetic energy, sliding surface can be cut off rapidly by the interaction of imbalance thrust and fissure water pressure. Then it can be converted into high-speed and long run-out sliding.