To mine deep resource of No.21 coal in Longgu Mine, and prolong the service life of mine, based on the water-resisting layer theory and hydrogeological data, it confirmed that there is water-resisting layer at the top of Ordovician limestone in Longgu Mine, and its thickness has been worked out. Besides, water-abundance characteristic of No. 14 and No. 15 limestone in Benxi Formation and its risk of water inrush is analyzed. The results show that more attention should be paid to collapse columns and faults, and the grouting reinforcement scheme is determined finally.