Abstract: Yanlong mining area is located in the west of Henan Province. In deep mining,No. 2 coal seam is threatened by karst fissure water of Ordovician strong aquifer,its mining safety is affected by increasing water pressure in the Ordovician limestone,the structure development of coal seam floor,the instability of water-barrier,etc. Based on the analysis of hydrogeological data and detection of water-rich floor by transient electromagnetic technology,the accurate calculation model of correction floor grouting depth was established,and the water disaster prevention technology of floor grouting reconstruction of No. 2 coal seam in Yanlong mining area was put forward,besides that,the industrial test was carried out. The results showed that three types of abnormal water-rich areas were in 12061 working face floor,where rock joints and fractures were more developed. The abnormal water-rich areas covered most of the working face floor,where water inrush could occur. The rhomboid borehole arrangement was beneficial to reduce the hole-spacing. The final floor grouting transformation depth was 65 m,the grouting pressure 9 MPa,the grouting diffusion radius 25 m. The aquifers of Ordovician limestone and the lower section of Taiyuan Formation could become a relative water barrier by grouting,which cut off the hydraulic connection between the floor Ordovician limestone aquifer and No. 2 coal seam. After floor was grouted,the water yield of the test hole was no more than 10 m ³ / h,the borehole wall was smooth,the original five abnormal water-rich areas disappeared,and the grouting transformation effect was remarkable.