Based on floor threat of Ordovician limestone water when mining 4#coal in Jiulong Mine, methods of underground borehole water injection and indoor numerical simulation were used to detect destroyed depth and study failure law in 4# coal. The results showed that the mining effect to the monitoring point occurred in a distance of about 66m, the more superficial monitoring point was, the farther distance would be; scope of mining influence was in the range of 20 ~ 55 m, greatest damage located in the front side of working face about 5 m and maximum damage depth was approximately 34 m.