Abstract: In order to study roof water inrush mechanism that the direct water filling source is weak water yield aquifer on coal seam, and explore the prevention and treatment method, based on 4 large-scale water inrushes during 1121 working face mining of Hongliu coal mine in Ningdong mine area, through theoretical analysis, numerical simulation and borehole drilling underground, it is concluded that the working face water inrush was induced by water in separated bed formed from overlying strata movement after coal mining. The key aquifuge of overlying strata was the necessary condition of water inrush from bed separation. The control function of periodic water inrush from secondary separated bed was researched based on the location, size and hydrogeology property of key aquifuge. The thickness of critical key aquifuge is 18 m, and the best time of water drainage is between bed separation water formed and periodic roof caved before water inrush occurred. The best position of water drainage is above the key aquifuge of coal seam in vertical direction and near the wall of lower tunnel in the plane for 1/6~1/3 of working face width. The research results were used in the similar condition mine widely.