Abstract: In the production process of the inclined coal seam open-pit mine, in order to reduce the stripping ratio and realize the internal discharge of the stripped material as soon as possible, and save operating costs, the mining method is changed from vertical mining to horizontal mining. At present, most economically reasonable or technically feasible methods are used to determine the width of open-pit mining areas. These two methods have their own advantages and disadvantages, and it is difficult to rationally optimize the width of open-pit mining areas. In order to optimize the width of the mining area of the open-pit mine using the horizontal mining and internal drainage mining program, a method for optimizing the width of the horizontal mining and internal drainage mining area was proposed. This method was based on the principle of open-pit mining, and established a horizontal mining internal drainage model for inclined coal seam open-pit mines. The relationship and change law between the width of the horizontal mining area and the average stripping ratio were analyzed. The influence of stripping transportation cost on the economical and reasonable stripping ratio and the relationship between the width of the horizontal mining area and the internal transportation distance of stripping were discussed. By establishing the relationship between the stripping transportation cost and the transportation distance, the relationship and change law of the width of the horizontal mining area and the economically reasonable stripping ratio were indirectly determined. Under the premise of stipulating the production capacity, the relationship between the width of the horizontal mining area and the annual advancement of the open pit was analyzed. Through the above methods, the economical and reasonable maximum width of the horizontal mining area was determined to meet the technically feasible minimum width of the horizontal mining area, and the optimization principle of the mining area width was given. This method and the determination principle were applied to Zhundong Open-pit Coal Mine, and the average stripping ratio, economical and reasonable stripping ratio and annual advance rate of open-pit mines with different horizontal mining area widths were analyzed and calculated. The results show that the optimized mining area width is 1.0 km, the average stripping ratio is 1.89 m3/t, and the economically reasonable stripping ratio is 2.11 m3/t, which ensures the mine′s production capacity and improves the economic benefits of the open-pit mine.