Abstract: The Xiaowan Power Station is the second highest arch dam in the world under construction. The height of the dam is about 292 m. The maximum excavated depth of dam foundation is 130 m. Large-scale excavation in the dam foundation of Xiaowan Power Station has brought intensive unloading phenomenon. The research purpose of this paper is to understand the rule and get mechanics model of unloading phenomenon. After excavation, the regularities can be found on landform characteristics and locations. In this paper the unloading phenomenon of Xiaowan is concluded into five types: step type, bend type, debark onion type, plate fracture type and floor heave type. Then the five types are analyzed by the methods of engineering geology and mechanics. Finally, four mechanical models are got ,and the unloading phenomena of Xiaowan can be explained reasonably by those models.