Many coal mines in China are greatly threatened by floor water inrush due to mining-induced fault activation. In this paper, a theoretical safety factor is proposed to assess fault activation during coal mining. Some critical effects on fault activation is analyzed, including fault dip angle, coal pillar width, stress concentration coefficient, and ratio of horizontal stress to vertical stress. Based on theoretical and numerical analysis, water in aquifer is very likely to burst into mining face along the fault F22 in 1000-m depth Xingdong coal mine. In addition, a new method of pre-cutting roof (PCR) is proposed to control fault activation. As the PCR angle increases, the shear stress along fault obviously decreases and the fault safety factor increases. The optimal PCR angles are greater than 75°, which is applied in the field. Field application shows that the water in aquifer did not flow into the mining face along fault F22 after PCR and the field-measured water levels in limestone-aotao is almost unchanged.