The underground powerhouse cavern in Baihetan Hydropower Station is in an extremely grand scale with complex geologic conditions, high initial ground stress, and high safety risks in excavation stage. The rules, characteristics and causes of surrounding rock deformation of left bank underground powerhouse were revealed by analyzing the safety monitoring data of the first to fourth floor excavation. Engineering measures to stablize surrounding rock were introduced. Results suggested that the deformation of surrounding rock during excavation was resulted from the combined actions of geological condition, geostress, and excavation. The total amount and depth of deformation of the crown influenced by geological structure were both large, and the deformation of the arch foot downstream of other tunnels were relatively large. Most of the deformation in the center of the crown and the upstream arch foot did not exceed 6.5 m, while the area beyond 11 m depth of downstream arch foot also witnessed some deformation. The deformation of the upstream and downstream platform was deeper than that of the crown. The spatial distribution and development of bolt stress and anchor bolt stress were consistent with those of surrounding rock deformation. There was still some space for bolt stress and anchor bolt stress in common tunnel sections. The present treatment measures for the platform layer have effectively improved the excavation and forming effect.