The geological condition and excavation shape of a high rock slope at the water-intake of a hydropower station are very complex. The faults, joints and fissures develop well in the rock mass and cut each other, which makes the engineering geological property for the rock mass to degenerate. The average excavation inclination is 88 and the maximal height difference is 106 m including vertical height part of 81 m from the dam crest to the bottom of the water-intake. The maximal horizontal depth of excavation is about 170 m. During the slope excavation, a series of deformation and fracture phenomena appear along the concrete surface covering the slope. They mainly includes cracks and surface bulging. Combining the engineering geological conditions and the monitoring data of the scene, a systematic analysis is made on the mechanism of deformation and fracture. The results indicate that the unloading fracture zone of the high rock slope especially in a high-intensive geo-stress environment is very significant in rock slope engineering. So, the fracture system is the normal response of the unloading and relaxation. Base on the analysis, the assessment is made on the slope stability. The analytical results show that the slope is stable at present.