One of the important problems for the Three Gorges Project is the stability for dam section No.1〜No.5 at left bank. The stability safety of dam section No.3 is studied with elasto — plastic Block Element Method (BKEM) in the paper. BKEM is a numerical analysis method. The basic unknown quantities of BKEM are the rigid displacements of the central points of block elements. Structure interface can be precisely and conveniently simulated in the method, and mechanical problem of discontinuous medium can be better. The correct ness of stability analysis of BKEM is proved with indoor model experiment and the two results are very consistent with each other. Finally, the result of stability analysis for dam section No.3 under several conditions is presented.