A stable slope is a crucial factor for water reservoir in a hydropower station regarding power generation. This paper mainly uses FLAC3D to analyze the slope stability and excavation with reinforcement support of stilling basin on the left bank of Kala Hydropower station regarding different operation cases. The numerical analysis reveals that the slope stability meets the design specification of class A and II slopes, while the potential instability area is mainly located in fragmented structural rock mass and class IV rock mass. After the excavation and reinforcement, the stability of the slope is improved to a certain extent, and Sericitolited Sand Slate is located at the foot of the slope, which basically has no effect on the slope stability. The slope unloading and toppling deformation of the left bank regarding the engineering area is obvious, and the shallow surface rock mass is prone to relaxation and damage during the slope excavation process. Therefore, it is recommended that the support should be on time, and according to the geological situation revealed, appropriate reinforcement treatment can be taken to ensure the safety and stability of the construction period. Sericitolited Sand Slate has a certain range of exposure during the excavation process, resulting in slightly higher slope deformation, so it should be carefully excavated. The slate is easy to soften when exposed to water, so attention should be paid to the timeliness and effectiveness of drainage measures during the construction period to avoid local collapse damage caused by softening of rock mass.