The authors use FLAC3D to carry out simulation analysis in underground power house of Ertan Project. FLAC3D is a three dimensional explicit finite-difference program. The authors consider the seepage factor which influence the stability of wall rocks and do lots of numerical simulation analysis with approximate three dimension. Systematical analysis of displacements of key points of side wall and systematical analysis of the plastic zone of the main cavern are carried out. The authors draw some disciplinarian conclusions from the systematical analysis. These conclusions will provide some useful reference for the study of large-scale underground cavern groups.