Abstract：The correct calculation formulas are established on the safety factor of slope by using analytical method. Two kinds of failure surfaces are considered in this study，planar failure surface and circular failure surface. In the case of planar failure surface，the analytical formulas are obtained for the critical slip surface and the minimum safety factor. In the case of circular failure surface，by performing minimization on the function derived，a set of algebraic equations involving two variables are obtained to determine the critical slip surface and the minimum safety factor. As an example，two alternative algebraic equations are given to determine the critical slip surface for the slope involving undrained cohesive soil with friction angle of zero. And the critical slip surfaces and the minimum safety factors are computed for some slopes.
