The impact of underground excavation on slope stability is controlled by many parameters, including the shape of slope, the mechanical property of soil and rock, the relative position of excavation zone and slip surface, and so on. The factor of safety (FOS) base on limit equilibrium method (LEM) and strength reduction method (SRM) is not suitable to evaluate the impact. Vector sum method (VSM) and orthogonal experiment are used to evaluate the impact by doing parameters sensitivity analysis. The result shows that the VSM could be used to in this research field, and the gradient of a slope, the relative position between a excavation area and a slope, the cohesion are the top three factors which impact the stability significantly.