Abstract: In order to achieve safe mining above confined water,stress field distribution of mining floor over confined water werecalculated and analyzed by FLAC~(3D).taking Xinji No.1 Coal Mine as the engineering background,and the mechanical criteria both for tensile failure and shear failure of the aquifuge were derived based on the theory of clamped beam.The results showed that breakdown point of the aquifuge located near the coal walland bearing capacity of the aquifuge increased with its thickness,tensile strength and shear strength while decreased with the inclined length of the coal face.Using this mechanical criteria,it wasknown that the floor aquifuge at one working face was apt to shear failure when water pressure was more than 1.78 MPa.Finally,water inrush prevention measures were proposed:firstly,to improve the bearing capacity of the floor near the coal wal; secondly,to shorten the inclined length of the working face according to confined water pressure.