As the lower permeability of the cut-off wall, the stress field and the seepage field of the slope would change after the cut-off wall is built. And the affect of the cut-off wall on the slope stability exists definitely. In order to analyze the mechanism of concrete cut-off wall working on the stability of the earth-rock dam, the strength reduction finite element method is adopted, in accordance with the ABAQUS. Besides, the difference and validity of the evaluating methods are analyzed by comparing to the three kinds of evaluating standard such as calculation, displacement and plastic through areas. The result reveals that the stability safety factor of the downstream slope increases while the upstream slope decreases after building a cut-off wall.