Aiming at rock mass deformation and failure character of slope, this paper mainly studies evaluating method of slope stability according to effect of combined underground and open pit extraction. Factor of underground mining was not considered in the past. Therefore, the conclusion was different from actual result. This paper relies on theory of random medium and starts from analysis of deformed stress of rock mass influenced by underground mining. Influencing feature of underground mining for slope rock mass is systematically analyzed. Then, evaluating theory and method for slope stability under combined mining effect is derived. By means of analysis result of theory and history-case, underground mining effect must be considered. Otherwise, result of calculation is not reasonable.