In order to improve the elfectiveness of early warning technology of water inrush from the coal seam floor in the working face of fully mechanized caving in ultra-thick coal seams, selecting No. 61303 working face of Tangjiahui Mine as the research area, the underground drilling layout and actual measurement were carried out , using the optical fiber sensing technology and high-density parallel electrical exploration technology, to observe the floor deformation and destruction of the 6th coal after mining. According to the change of the optical fiber strain nephogram of the observation hole, when the working face does not reach the orifice position of the monitoring section , the bottom plate is mainly affected by the mining advance stress in the early stage. When the working face pushes through the monitoring borehole , due to the bottom bulge expansion caused by the bottom plate pressure relief and the shear force, it presents a certain tensile strain. Combined with the change of apparent resistivity , the development of bottom plate crushing and its control horizon were analyzed. The depth of the failure zone of the floor rock stratum of No. 61303 working face is 14.5 m, which is located in the middle of the sandstone mudstone layer. The maximum depth of failure disturbance is 32.0 m, which is located in the lower part of sandstone mudstone layer.