Abstract: The discussion focuses on the viewpoint made by Song et. al. in the article "On the inclination angle of undrained shear slip surface in saturated soils" that the inclination angle of the slip surface under undrained conditions of saturated soil is 45°. According to the principle of effective stress, the strength and failure of soil are determined by the effective stress acting on the soil skeleton. According to the Mohr-Coulomb strength theory, when the major principal stress directions are respectively vertical and horizontal, this inclination angle should be 45°±φ′/2. A large number of experimental results and engineering practices have also proved that 45°±φ′/2 is basically in line with reality. Some factors affecting the inclination angle of the slip surface under undrained conditions of saturated soil are discussed, but none of these factors would result in such a large gap between 45° and 45°±φ′/2. It is particularly pointed out that in the limit equilibrium analysis and numerical calculation of saturated soil, the analysis object can be either the saturated soil mass or the soil skeleton. In the former, the main mechanical properties of the material are φ = 0° and dε_v = 0, at which point the Mohr-Coulomb strength theory will degenerate into the Tresca strength criterion, and soil mechanics will degenerate into material mechanics. In the latter, the main mechanical properties are the angle of internal friction φ′, compressibility and dilatancy (contraction), and at this time, dε_v=0 is only the boundary condition of the analysis. Finally, a triaxial undrained test of saturated sand is conducted, and the results show that the inclination angle of the slip surface is basically 45°+φ′/2.