Abstract: The rock mass forming the dam foundation of Xiao Wan hydroelectric station during excavation revealed a series of deformation and fracture phenomena. It mainly behaved as a split plate and surface rock bursts, and cracks along the existing structure, expansion and dislocation, and so on. On the basis of the analyses of the above-mentioned phenomena, this paper studies the geological mechanic response of the rock mass due to the dam foundation excavation in the high in-situ stress environment. It is found that the rock mass deformation during excavation is mainly concentrated on the shallow surface of the dam foundation, which has time effect characteristics. According to analyses of deformation-destruction phenomenon and the study of deformation responses, the mechanism of deformation-destruction is understood. The mechanism applies to the excavation of the hard rock river bed of the dam foundation in the high in-situ stress environment. Shallow and superficial transformation result from the stress redistributed during down-cutting of river and unloading of excavation. Ultimately, it can be summarized that the deformation and fracture of river bed are a pressure rupture-unloading rebound model, and the pressure-rupture is the main type.