Abstract：Xiaowan Hydropower Station is a huge powerstation located in the middle-lower reach of Lancangjiang river. It is a double-curvature arc concrete dam with a maximum height of 292 m，the second highest arch dam under construction in the world. Owning to the narrow，high and steep valley as well as the higher geostress，strong unloading，the development in the dam¢s base during excavation is discussed which worsens the rock mass and leads to a series of troubles. The geological conditions of the dam site，the macroscopical characteristic of the unloading fissures of the base are considered and it finds that：(1) the dip angle of the unloading fissures changes from big to small gradually from the top to the bottom；(2) the unloading fissures in the both banks are inversed in dip direction；(3) the surface of the unloading fissures is almost parallel to the surface of excavation base. Meanwhile，the unloading fissures are classified into three classes as successive fissures，extensive fissures and fresh fissures. Finally，the distribution pattern and scope of the unloading fissures are analyzed. All these are meaningful to understand excavation unloading problem as well as to choose suitable measures to strengthen rock masses.
