Abstract: The safe mining of under-group coal seam in Yangcun coal mine has been seriously threatened by confined water under floor.Therefore,the study of failure depth of mining floor is the key to evaluate the water-resisting ability of floor.Based on the hydrogeological conditions at 4602 working face in Yangcun coal mine,the engineering geological model of under-group coal seam floor was set up.Studying on the deformation and failure laws of coal seam floor mining by FLAC^3D,the maximum failure depth of floor is about 12 m under nomal mining condition.Compared with the working face floor measured results,the two results are in agreement.The results provide reference for rationally mining of under-group coal seam and calculating the indexs of water-resisting ability of floor in Yangcun coal mine