In the engineering of soils stability, evaluation of earth pressure, foundation bearing capacity and slope stability are all related to limit equilibrium conditions. Limit equilibrium conditions of a soil element are extended to limit equilibrium conditions of a slip surface. Necessary and sufficient conditions required for determining the limit equilibrium state is proven. Based on the extension, a stability method combining limit equilibrium analysis with finite element stress analysis to provide a factor of safety is proposed. And the physical interpretation of the definition of the factor of safety is offered. Then several representative two-dimensional examples about soils stability are analyzed. The results show that the method could be used to analyze the stability of geotechnical structures.