Abstract: A unstable rock block has been identified at the Xilongchi reservoir. It is named as the BW2 dangerous rock block because it threatens the safety of the reservoir. It was cut by F118 fault into column. The paper presents a systematical and detailed description of the site engineering geological conditions. Based on the analysis, it proposes the possible failure models. The Fast Lagrangian Analysis of Continua method （FLAC3D） and the limit equilibrium method are applied to forecast the deformation and stability of the BW2 dangerous rock. The treatment measures are suggested accordingly.