Abstract: In order to effectively predict the floor water inrush under the pressurized mining in the working face, based on No.15 coal seam in a coal mine as a study background. The failure depth of the floor is calculated by using the plastic theory and the empirical formula. Considering the influence of mining and occurrence factors, and based on the modified water inrush coefficient method, the formula of limit water inrush coefficient is put forward to determine the reasonable working face size and improve the accuracy of water inrush prediction.The water inrush of the floor is explored by site construction hydrological drilling, physical and mechanical properties of floor resisting stratum measured by the rock core，and comparison with theoretical calculation results. The possibility of water inrush from the floor of 15# coal seam is effectively predicted. The results show that: The main water-resisting layer of floor belongs to medium hard rock, which have water separation performance. The water content of the floor is weak, and there is no condition for water inrush from the bottom plate. The calculation results of ultimate water inrush coefficient show that there is no risk of water inrush in the complete section of the floor of the coal seam, but in the structure damaged fault section have. It is suggested to reduce the size of working face properly, find out the geological characteristics of the floor, and strengthen the water exploration and drainage in the weak areas such as the structure to prevent the water inrush risk.