Abstract: Based on the shear strength of cohesive soil, the shear strength expression of soil divided by the safety factor is incontradiction with the Coulomb criterion. Only the frictional resistance of the shear strength expression is reduced in the improved expression, thus constructing a new strength reduction formula for cohesive soil which conforms to the Coulomb criterion. In this way, a new slope stability factor expression is derived. Then the expression is used to calculate the safety factors of seventeen soil slopes. The results show that the values of safety factor of soil slopes calculated with the new expression are 0.22%~25.05% smaller than those in the current textbooks, which is safer for engineering projects. In comparison,the proposed new safety factor expression of soil slope is more reasonable. This improved expression has reference value for related engineering analysis.