Abstract: To study the control mechanism of large-section open-off cut roof, No. 5205 open-off cut in a coal mine is selected as an engineering case. The deviatoric stress of roof, fracture field and deformation law are investigated by UDEC4.0 in the change process of width from 6 to 10 m. The results show that: (1) The deviatoric stress of roof gradually increases in shallow part and decreases in deep part. The wider the open-off cut, the lower the deviatoric stress in shallow part and the higher in deep part and the same time the peak of deviatoric stress is moving to the deeper part; (2) The fracture fild in surrounding rock were divided into three areas: fracture transfixion area, fracture area and microfracture area, which distributed as semi-ellipse; (3) With the width of open-off cut increasing, The trend of microfracture area transformed to fracture area and fracture area transformed to fracture transfixion area are strengthed in the roof. Besides, the roof subsidence is aggravated. It believed that: (1) The stability of unstable triangle block is the more important to keep roof safe; (2) High-performance bolt can control fracture slip preferably; (3) Double-truss and anchor rope can be anchored in the shoulder non-fracture area and deep roof pre-stressed superimposed area, which can control the expansion and deformation of fracture transfixion area in the middle part of the roof. Based on this, the combined support technology of high-strength and high pre-tension bolt& net &bar and double-truss and anchor rope is proposed. The observed results of mining pressures show that the open-off cut is self-stabilized after it is dug out for 10 days; the relative convergence of roof-to-floor is 128 mm; the relative convergence of both sides is 96 mm; and the separation of roof is 4 mm.