Geological defects always exist in natural foundation more or less. The foundation treatment is a significant component in dam construction. Based on three-dimension finite element method (FEM), the failure process of dam foundation with weak interlayer is studied with overload method and strength reduction method respectively, the quantitative results of the foundation safety are gotten by comparing with before and after foundation treatment to show the treatment effect. Comparing the treatment effect of each design schemes, the optimum design scheme is obtained. The calculation results show that the quantitative results of the foundation safety can be computed by the finite element method, so the approach in question is proved to have obvious advantages.