The ranking grade of rock engineering stability is generally determined by the maximum membership degree principle, and the value of membership degree is an exact numerical value. However, the membership degree of uncertainty with randomness and fuzziness has not been considered. The membership degree with uncertainty can describe the uncertainty effects of the evaluating factors on rock engineering stability and fully shows the degrees of one evaluating factor belonging to the ranking grades. In this study, a quantitative index representing the membership degree with uncertainty is proposed and obtained based on the membership degree of the cloud model. The membership degree of the cloud model is obtained based on normal cloud theory, the system and the ranking grade of evaluation factors and analytical hierarchy process (AHP) based on scale of the cloud model. An evaluation method for the membership degree of cloud model is conducted and applied in a rock slope project. The feasible and practical evaluation results is verified by comparing with that of AHP method, the extension theory and the numerical simulation. This proposed method can serve as a decision tool for stability evaluation in other similar projects.