Abstract: For solving the support problems of thin crushing roof when tunneling the large section open-off cut at the second time in 1001 fully mechanized caving face of one certain coal mine, base on the "dispersion-block" model of overlying strata, it built UDEC discrete element numerical calculation model of open-off cut in lower coal seam of the goaf, drew the stress and displacement curve of surrounding rock of the large section open-off cut. The numerical simulation results indicated that the roof rock stress was more concentrated in front of coal wall region of 2.5 to 4 m after finishing the first horizontal excavation. In order to ensure effective control for roadway roof during second tunneling of open-off cut, it was necessary to focus on strengthening this region. Combined with the actual geology and engineering condition, it used malisan grouting technology to reinforce the roof of this region.