Based on the limit equilibrium method and the reliability theory, the probability distribution model of safety factors is constructed under consideration of the variability of controlling factors for slope stability. According to the practical engineering characteristics, comparing random safety factors with the critical safety factor to build the security region and failure region, and multiplying the average value of safety factors located in the security region by the slope reliability, we obtained the effective safety factor based on the random dual index. We also achieved the changing characteristics of the mean safety factor, the average value of safety factors, the reliability safety factor and the effective safety factor in different variability of slope stability’s controlling factors by comparison of examples. The example shows that the new randomized evaluation index of slope stability proposed in this paper associates the macroscopic characteristics phenomena of slopes with quantitative analysis, and thus provides a reference for similar slope projects.