Abstract：After impoundment of reservoir，there would be a large amount of hydrous slopes，with stability being influenced of the change of the water level. As a common practice，the present slope stability analysis would begin with the determination of phreatic surface，and then the stability would be analyzed. Therefore whether the approach to locate the phreatic surface is appropriate or not is essential to the accuracy of the stability analysis. However the method of roughly generalizing the phreatic surface of the bank slope through experience is commonly adopted，so the correct calculation method is an urgent problem. In this paper，the phreatic surface is calculated through seepage calculation and generalized through experience respectively. Then the effects of these two methods on the stability analysis are compared and analyzed to determine the inaccurately of the method through experience. From the calculation example，the inaccuracy rate through experience is above 10%.
