Abstract：The relationship between the factor of safety of unsaturated soils slope and the stability is proposed for a given unsaturated soil slice. If the minimum factor of safety is greater than or equals to the allowable factor of safety，the slice is safe. The general limit equilibrium methods for stability analysis，Swedish slice method and Bishop method，are conducted based on the ratio of factor of safety and the improved Mohr-Coulomb failure theory. Several special formulas are achieved when the pore pressure is 0. The minimum factor of safety of an unsaturated soil slope is analyzed with the changes of different parameters. Some results show that with the increase of the ratio of factor of safety，the minimum factor of safety significantly decreases，yet the density has little effect on the minimum factor of safety.
