The stability analysis of multi-zigzag plane base surface of concrete gravity dam is studied. For the equation specified in Gravity Dam Design Specification is not able to resolve the stability of three-dimensional base surface

a new limit equilibrium quasi-safety factor formula is educed for stability assessment

based on nonlinear finite element method （FEM） results. And the strength accumulation coefficient method is also adopted for investigation of damage model and safety checking. In those two methods the material parameters and loads are treated with each partial coefficient. Those two methods are suggested to use together for the base surface stability and an engineering project is given as example. The computation results show that these two methods are reasonable and convenient for the stability assessment of multi-zigzag plane base surface.