Based on special case that the 9# coal mining is under double threat of the thick-hard roof and Ordovician limestone water inrush from floor, a survey and research on the 9# coal floor failure law is conducted with on-site water injection test and numerical simulation. On-site water injection test indicates that different degrees of damage occurred within the scope of monitoring of working face coal seam floor and the max failure depth is up to 18.80~21.00 m; and the distance between 9# coal and Ordovician limestone aquifer is less than 20 m; Simulation results show that,as for the degree of damage of coal seam floor under thick-hard roof,forced carving is more destructive than filling mining.