Abstract: In accordance with the case of large mining depth in Xingdong coal mine,study on coal floor strata failure depth test of working face 2121 was carried out by water injection test.According to variation of unit injection quantity and distance between testing boreholes and working face advance,coal floor strata failure depth was determined by continual water injection and intermitted monitoring.It was shown that coal seam floor failure depth of working face 2121 was 32.5 to 5 m,approximately three times as deep as that of shallow working face(in genera l,9.15 to 12.0 m).Coal seam floor failure depth increased with mining depth.Potential risk of water inrush through coal seam floor of working face 2121 is much higher for deep mining than for shallow mining.Testing results provided scientific basis for formulating water prevention and control program for deep mining of Xingdong coal mine.