Abstract: The appropriate setting width of the waterproof coal pillars is an important measure to prevent water inrush hazard during near fault mining. The working face 123_up01(south) of Jining No.3 Coal Mine in Shandong Province is adjacent to F8 fault in the west. The fall of F8 fault is large, which may be conducted with the Ordovician limestone aquifer at the bottom. To ensure the safe mining in the working face, it is necessary to calculate the size of waterproof coal pillars of fault F8. In view of the actual occurrence of coal strata, the spatial relationship between fault plane and coal seam occurrence, and the threat of water inrush caused by the confined water under the coal seam floor, the method to calculate the waterproof coal pillar size of water-conducting faults in Detailed Rules for Water Prevention and Control in Coal Mines was improved, and the formula of the head height from the water level to H_a at the vertical foot of the fault plane and the improved formula of waterproof coal pillar size were deduced. By comparing the calculation results before and after improvement, it is found that the original calculation formula in Detailed Rules for Water Prevention and Control in Coal Mines idealizes the coal strata as horizontal strata, and takes the smaller water pressure value of coal floors, the coal pillar size of F8 fault is calculated as 112 m, while the calculation result of the improved formula is 128.5 m. Since the improved formula takes into account the spatial relationship between the actual occurrence of coal strata and fault plane, and the position where the water pressure value obtained is accurate, so the calculation result is more accurate, which provides a more scientific reference for safe coal mining.