Abstract: Regarding to the inclination angle of undrained shear slip surface in saturated soils, there are different opinions in the profession. This issue is explored through numerical calculations and theoretical analyses, and it is made clear that the inclination angle of slip surface in saturated soil at any point under undrained shear is theoretically 45 degrees, and attempts are made to explain this phenomenon. It is pointed out that the slip surface is not necessarily the surface with the highest shear stress ratio, and for undrained shear of saturated soil, its strength is determined by the effective stress, but deformation should be determined by the constitutive total stress, i.e. the sum of the effective stress and the excess pore water pressure. In other words, the inclination angle of the slip surface in saturated soil at any point on undrained shear is determined by the friction angle corresponding to the constitutive total stress. In addition, the difference and connection between the slip surface discussed here and the shear band discussed much more in literature are briefly stated, and opinions are given with regard to the possibility of the latter appearing in practical engineering.