Based on the method of slope stability assessment, the disadvantages of the slope stability assessment method has been analyzed. The factors of approach for evaluating rock slopes' stability have been proposed, and the factors that affecting the slope stability have been analyzed and categorized. They have been categorized into three groups, which are geological factor, environmental factor, and engineering factor. Based on these, a new approach for evaluating rock slopes' stability under the comprehensive effect of multiple factors combining Analytic Hierarchy Process (AHP) and fuzzy mathematics has been set up. A certain open-pit's rock slope has been evaluated by this approach. The results under the new classification system of slope stability classification showed that the central and southern district of footwall slope was potentially unstable areas, which need further analysis. Moreover the slope stability of the hanging wall was much better than the footwall, which was similar with CSMR classification results. The results showed that the factors' categorization was rational and the assessment was valid.