Abstract: The excavation and stability of hundred meters high rock slopes in water conservancy project construction are of important technical issues . This paper bases on the abundant geological investigation of a construction site and examines the associated cut slope with height of 330 m in a large Hydroelectric Station in southwest China. The reactive deformation of the high rock slope with great sloping ratio is studied under the condition of complex geological conditions and high in-situ stresses. The study is systematical. The basic laws and special characteristics of the deformation responses are acquired and can be classified as follows: (1) During the slope excavation, there is a strong synchronism between the slope deformation and its excavation process. (2) The high slope deformation is mainly influenced by excavation. (3) The slope deformation generally is weakened as increase of the distance between the excavation plane and the monitoring position. Immediately after its formation, the cut slope had small deformation and the deformed depth was also shallow. After having been excavated for 12 to 18 months, the cut slope was stable. The findings are helpful to the design and construction of similar high cut slopes.