The excavation of large underground powerhouses is of great scale, and stability problems of the surrounding rock are always involved during the excavation. When a developed alteration zone is close to the excavation site, the large deformation risk of the surrounding rock could increase sharply. Here, the main powerhouse excavation project of Fengning Hydropower Station was taken as the research object, the deformation features and mechanism of its surrounding rock with an alteration zone were analyzed based on the geological conditions and monitoring data. The monitoring results show that the deformation of side walls was generally large, with multiple monitoring values exceeding 50 mm. Larger excavation area induced greater deformation. The time-dependent deformation of the surrounding rock was obvious under the influence of the alteration zone. The analysis indicates that the surrounding rock deformation in Fengning main powerhouse was affected by the alteration zone, large-scale excavation and in situ stress, because the alteration zone, which is widely distributed along the rock joints and faults, weakened the rock mass quality and became the internal cause of large deformations. Furthermore, the unloading damage of the surrounding rock caused by large-scale excavation created the condition for large deformation and the major principal stress of in situ stress intersecting with the longitudinal powerhouse axis at a large angle magnified the adverse effect of the former two factors, which eventually caused the large deformation.