To further understand the failure mechanism of water inrush from mining face floor, the water inrush failure mechanism of mining floor is analyzed using the Mohr diagram and fracture mechanics theory. By considering the effect of saturated pressure on the branching crack tip, an analytical formula for the plastic fractured region and a damage threshold of rock mass failure are established with damage fracture mechanics and the unified strength theory. An equation of damage fracture energy is determined by coupling crack propagation and rock damage, and the factors influencing the damage fracture energy are analyzed in depth. The results show that the stress intensity factor (SIF) of the crack tip at the complete unloading state of the maximum principal stress is higher than that at the biaxial stress state with a confining pressure coefficient 0.5, in which the mining floor has been found to be more prone to failure. By taking into account the influence of the plastic fractured region of the branching crack tip, it is noted that the crack damage fracture energy tends to be higher and water inrush risk of mining floor is more obvious. There is a positive correlation between and the crack half-length a, the ratio of crack connected area to the crack total area , crack seepage pressure and the minimum principal stress . Whereas it shows a negative correlation with the friction coefficient of crack surface and modulus of rock mass . The results provide some insights into the prediction of water inrush failure mechanism of mining floor.