Abstract: The stability analysis and average slope angle value of open-pit mining slopes are key issues that are in mine slope design. In mining engineering, with mining progresses, the slope height gradually increases will lead to a change of the slope stability, and the average slope angle needs to be continuously adjusted. This paper combines specific mine slopes, and geological survey points on site that were arranged to investigate the engineering geological characteristics of the rock mass, and obtain the rock mass classification (RMR) values of the rock mass quality indices combined with indoor testing. According to the rock mass classification values and the terrain of the mine, the rock mass classification values range and slope height range of this study are determined. During analyzing the stability of mine slopes, the geological strength index (GSI) determined by the rock mass classification value, and the strength parameters of the rock mass were estimated by the slope height are determined firstly. Then the slope stability was calculated by simplifying the bishop method to obtain the average slope angle under the conditions of the given safety factor. By analyzing the influence of rock mass classification value and slope height on the average slope angle, a theoretical calculation formula for the average slope angle was proposed, and a stability assessment diagram for rock slopes has been drawn. The calculation results of the research conclusions are compared with the calculation results of other scholars‘methods to verify the reliability of the research conclusions by combining a slope engineering example of a limestone mine in the southern Jiangsu region. The research conclusion of this article can provide a theoretical basis for the design of mining slopes and the stability analysis of open-pit mining slopes.

Key words: RMR Method , average slope angle , slope height , open-pit mining