Abstract: In near-fault coal seam mining, the activation of the fault to communicate with the aquifer often induces mine water accidents and affects the safe production of coal mines, therefore, the width of the coal rock pillar is of great significance to the prevention and control of water accident in the mine fault. Coal Mine Water Prevention and Control Regulations have elaborated in detail on the retention of coal and rock pillars in various situations, and given the formula for the retention of coal and rock pillars in the case of water-conducting faults, but in the face of the threat of complex pressurised water under the faults, the existing formula for calculating the anti-water-isolation coal (rock) pillars is not perfect enough, and the results of the calculations are not precise enough. In order to optimize the width of the coal pillar, for the spatial relationship between the water-conducting fault and the coal seam and the water-conducting mechanism of pressurized water, combined with the hydrodynamic analysis of pressurized water conductivity, we established a fissure water movement model according to the constant total energy equation, analyzed the relationship between seepage velocity, pressure and energy loss in the process of water conductivity of the fault, and deduced the formula for calculating the hydraulic pressure of the pressurized water in the channel of the fault to an arbitrary position. The calculation formula of water pressure in the fault channel is deduced, and the calculation formula of water pressure in the fault channel is then optimised for the waterproof coal rock column under the condition of fault water conduction in the Coal Mine Water Prevention and Control Regulations. Taking F7202W working face near DF3 fault of Zhaizhen coal mine as an engineering example, the formula before and after optimisation was used to calculate the width of coal pillar, and the comparison of the calculation results showed that: the original calculation formula of the Coal Mine Water Prevention and Control Regulations didn't take into account the water pressure influence of the pressurised water conduction and rise, and the water pressure of the bottom plate of the coal seam was idealised and water pressure was valued according to the position of the bottom plate of the coal seam, and the size of the pillar of coal rock on the DF3 fault was 117 m. The optimised calculation method combined with the actual water breakout data of the Austro-Ash aquifer in each working face of the seven mining areas to calculate the stable flow rate of each working face, and based on the maximum stable flow rate of the water breakout in the Austro-Ash aquifer, the optimised formula calculates the size of the water-proof pillar to be 108 m. A three-dimensional fluid-solid coupling numerical calculation model was established through COMSOL Multiphysics to simulate the mining process of the F7202W working face in Zhai Town Coal Mine. Based on the distribution patterns of stress in the roadway and water pressure in the fault, as well as the variation of fault permeability under different water-proof coal pillars, the safety and rationality of the 108 m width of the coal pillar calculated by the optimized formula were further verified. Compared with the size of the safe water-proof coal pillar calculated by the Coal Mine Water Prevention and Control Regulations, the width was shortened by 9 m, increasing the recoverable coal resources. The research results provide a reference for the precise calculation of the width of the coal and rock pillar for fault water prevention and separation under different water pressure conditions in mine exploitation.