Abstract: In order to study the failure mechanism of floor mining above confined water, the floor is simplified to the mechanical model of clamped-edge rectangular thin plank, which bears even load on the upper surface. Basic theory of elasticity thin plank is utilized, floor deforming destruction and strained condition are analyzed, the floor heave of roadway formula is deduced, and the maximum floor heave and the position of stress are obtained, which are related to the elastic modulus, the thickness and length and width of the floor, the Poisson's ratio and unit weight. According to the compressive strength of floor, it's able to judge whether the floor is damage, and the center point of long side begins to destroy. At the same time, according to theory of elasticity thin plank, we can get the size and direction of the main moment, and provide theoretical basis for floor damage control measures. We obtain the damage area and form through the MATLAB software. The three dimensional distribution map of deflection horizontal stress and shear stress of the floor are drawn.