The determination of positions of critical slip surfaces is a hot spot of research on geotechnical engineering. In two-dimensional slope stability analysis applied with finite element-strength reduction, the points with the maximum equivalent plastic strain along depth are mostly on the critical slip surface for a slope at limit equilibrium state. One set of wavy signal function consisting of points with the maximum equivalent strain can be constructed by deploying a series of parallel lines approximately perpendicular to the slope surface. The wavelet analysis smoothens these points on the critical slip surface. Through the classic example analysis and comparisons with Spencer’s method and other previous research, the results show that the method in this paper is reasonable and effective, and can be used to locate the critical slip surface.