Abstract: The maximum and minimum principles in the slope stability analysis,can be used to test the accuracy of the stability analysis model,and obtain the related condition used in improving the model. This paper uses the Simplified Threedimensional Sarma Method. It analyzes the variation of the stability factor caused by the direction of shearing force on the sliding surface .The results show that no matter what the distribution of the direction,when it changes,the corresponding stability factors has a great value. This just proves that the maximum principle in threedimensional slope stability analysis is applicable. At the same time,on the base of a rigorous determining method for the twodimensional potential sliding surface of slope,this paper builds an algorithm for determining the threedimensional potential sliding surface,and analyzes the effect of the parameters in the object function on the stability factor through the case in ZhangXings paper. The results show that twodimensional determining method is feasible,and it also proves that the minimum principle in the twodimensional slope stability analysis is met. But in the threedimensional slope stability,the results have some difference with the demand of the minimum principle for the threedimensional limit equilibrium method not taking into account the boundary conditions of threedimensional slope．