Abstract: To solve the safety problems of gas’s concentration exceeding limits at the large mining height coal working face and its upper corner,the effect of mining height on roof unloading and gas flow channel damage evolution in goaf was analyzed based on the theories of unloading rock mass mechanics. Combining with damage mechanics,the relation- ships between damage factor,unloading percentage and permeability were built. And the roof unloading and gas flow channel damage evolution laws of goaf in different mining height working faces were calculated by the discrete element method. Meanwhile,the unloading damage scope of gas flow channel in different mining heights was analyzed according to the relationship between effective stress and permeability after unloading. Then it provided a gas drainage technology of directional boreholes with large diameter in the middle-high fracture zone by using favorable channel formed in large mining height mining,and it verified the cracks connect and development of gas flow channel. The results have shown that roof unloading degree increases nonlinearly with the increase of mining height;the larger the mining height,the greater unloading percentage and the damage factor of roof,the more generated fractures,then the position of roof per- meability catastrophe point and gas flow channel development are much higher. Drainage flow velocity using 153 mm diameter borehole is about 2-3 times more than that using 96 mm diameter,and the drainage concentration of bore- holes in the middle-high zone of gas flow channel is about 2. 4 times higher than that in middle-low zone.