Abstract: Only the effective water-resisting zone has water-resisting capacity. At present about 61 percent coal reserve in Feicheng coal field is suffering the danger of water-inrush from floor Ordovician karst water while it is mined. Therefore monitor the broken floor depth is a main research project of controlling mine inundation in Feicheng coal field. The being mined coal seam 8 and coal seam 9 of Caozhuang coal mine in Feicheng coal field is selected as the monitor objects. The broken floor can be monitored with infusion method. The stratum original fracture degree before mining coal and broken depth caused by underground pressure can be monitored with the patent ‘Double Ends Sealed Water-loss Monitor’. The research result show that the depth of the broken floor caused by mining coal seam 8 is 36.5 m and the depth of the broken floor caused by mining coal seam 9 is 14.2 m.