Abstract: Confined water inrush caused by fault activation induced by coal mining under high in-situ stress is one of the main forms of water damage in deep mining. Reserving fault waterproof coal pillar is still the most important method to prevent fault activation and water diversion. We take F₂₂ high angle normal fault in Qianyingzi Coal Mine as a typical example, based on the existing drilling, geophysical and roadway exploration data, the specific hydro-geological and engineering geological conditions of the coal mine, and according to the field test data, the finite difference numerical analysis method was used to study the width of the waterproof coal pillar under the full mining of the W3221 working face near F₂₂ fault of Qianyingzi Coal Mine; at the same time, the width of the F₂₂ fault waterproof coal pillar was calculated and verified according to the theoretical formula. The study shows that when the coal pillar width is less than 40 m, the stress and displacement of the fault zone have changed significantly, and the plastic zone in the fault zone has communicated with the water-conducting fracture zone of the coal seam roof. The comprehensive analysis of numerical simulation and theoretical calculation results show that the waterproof coal pillar with a width of 40 m can effectively prevent the fault activation and ensure the safe production of the working face.