To further understand the failure mechanism of mining face floor in the case of coal mining above confined aquifer, the scope of plastic failure in the yield zone of mining face was analyzed by using fracture mechanics theory and Griffith criterion. On this basis, the mechanical model of the plastic failure scope of mining floor was established in line with the slip-line field theory. A theoretical solution for the plastic failure scope and the maximum floor damage depth was given by the model. The relationship between the maximum depth of broken floor and its affecting factors was analyzed. It could provide a theoretical basis for preventing water inrush above a confined aquifer. Finally, the theoretical solution was proved to be feasible by an engineering application example.