Abstract: Based on a lot of detailed hydrogeological work in working face 15423N in Jiulong Coal Mine, it studied the hydrogeological conditions of this mine and working face 15423N. It studied and obtained that the small concealed collapsed column under this face floor, which shortened the distance between the Ordovician aquifer and this face coal seam floor impermeable layer by 68.5 m. The water pressure of this face coal seam floor in the Ordovician aquifer and Daqing aquifer was high, the strength of remaining impermeable layer floor was insufficient, and it could not stand the high pressure of Ordovician aquifer. Collapse column insidious, scale-range and horizontal hydraulic connection limitations were the major causes of the goaf lag floor water-inrush. It was another cause of the floor water-inrush that the deficiencies and deviations of hydrogeological understanding for Ordovician aquifer under closed stagnation conditions.