In this paper, the deficiency on present mechanism of water inrush from floor is analyzed and it is found that water inrush from fault not only relative with static loads, but also with dynamic loads, especially faults in deep are easily acted by low dynamic loads. Based on fracture mechanics, the fault activation model is built and the stress intensity factor is calculated in view of the deficiency on present mechanism of water inrush from floor. The influence on fault depth, fault length, fault span, fault thickness, underground water pressure is considered in the model for faults in deep. Whether the fault is active is judged by strain energy density factor. The strain energy density factor is relative with fault parameters. It is reverse with fault thickness, positively increases with fault depth and stope span. It also positively increases with dynamic loads and acting time.