Abstract: On the process of stripping the roof rock in No.6 coal seam to the middle and southern part of stope, we found the No.6 coal seam missing suddenly. After the investigation, analysis of stratum of the disclosed benches, we found that the reason for No. 6 coal seam missing is a new graben has formed in this area. With deep analysis, we found this graben is the F₃ reverse fault of Heidaigou Open-pit Mine extending to Haerwusu Open-pit Mine, and then we accurately complement the geological report relating to the F₃ reverse fault. By the analysis of this essay, we believed that the area in the Heidaigou Open-pit mine is also graben, with larger area than the one in Haerwusu Open-pit Mine.