Abstract: The combined model of open-pit with underground mine has always been adopted by metal mine and inclined coal seam mine before, and we used the method of open-pit or underground mining to mine the shallow horizontal thick coal seam previously. Recent years, in China's major coal production base Pingshuo mining area, the method of combining caving mine and open-pit method was adopted to exploit the shallow horizontal thick coal seam, and it has already made considerable economic benefits. In the paper, through two methods of numerical simulation and similar simulation, the failure problems of the open-pit slope under the condition of combining open-pit mining with underground mining were analyzed and discussed, results showed that the failure of the open-pit slope is a gradual process, in order to guarantee the normal open-pit mine production works well, the explored boundary must be controlled well by the vertical stature of safe mining.