In order to study the deformation and damage of the surrounding rock of the left bank underground powerhouse of Baihetan Hydropower Station, through the field investigation of the main powerhouse，the primary destruction types are classified as rib spalling,bellying of surrounding; creepage and the precipitating of calcium. A calculation model was established to study the damage mechanism of the left bank underground powerhouse during excavation unloading. The result indicated that the geostress of the left bank underground powerhouse redistributes under the influence of the excavation, which results in several stress concentration at the roof of arch, spandrel and the sidewall of upside main powerhouse where the major principal stress exceeded critical value, leading to various damages of the surrounding rock. There were many long cracks with poor mechanical properties and weak carrying capacity, which will trigger bellying of surrounding, creepage and the precipitating of calcium in the upstream of the main powerhouse under excavation. And the cracks will induce abnormal increase of geostress in close region and rib spalling of the surrounding rock. The stability of the surrounding rock of the upstream should be paid close attention.