Abstract: In the flood season of 2005, because of upstream flood, the water level of Longyangxia reservoir went to the height of 2 597.62 m, approaching normal high water level for the first time since operation. By analysis on observation data of dam and damfoundation displacement, this paper explained the phenomenon of unclosed water level component of the radial displacement of arch crown, summarized the rules of damdisplacement and got to the conclusion that the damwas normal in operation.