Abstract: The waterproof coal pillar is the main technical measure to prevent roof water inrush. The water inrush accident of loose aquifer occurred in theⅠ03(2) working face of Laogongyingzi Coal Mine when the waterproof pillar size met the regulation requirement, and the largest water inflow was 760 m³/h. From the strata lithology and structure, the loose aquifer pressure and the effective thickness of water-resisting layer, the water inrush mechanism is analyzed, the conclusion is that water inrush happens under the special hydrogeological and mining conditions. Based on the ratio of the minimum bedrock pillar thickness to the coal seam mining thickness in working faces that had been mined, the new method is proposed to determine waterproof pillar size. When the ratio value is bigger than 29, the mining process is safe, which is verified in the subsequent working faces.