Taking a certain slope of Sichuan-Tibet Highway as an example, this study simulated the slope excavation and analyzed the stability before and after its excavation. According to engineering geology investigation on the spot and the relevant data, the original landforms before the highway's building appeared again, thereby the calculated model being developed. Fast lagrangian analysis of continua method was employed to simulate the distribution of internal stress, the change of displacement and the course of slope instability. The results of strength reduction method showed that the excavation of roadcut was the main cause of slope instability.