Abstract：Theoretical analysis，simulation experiment and engineering exploration are adopted to study the impact of the location of primary key stratum(PKS) on the evolution range of water flowing fracture. The final result shows that，as the distance from PKS location to coal seam is shorter than a certain value，the fracture extends to the top of bedrock，while the height of water flowing fracture is obviously larger than its value calculated by the conventional estimation method based on “Regulations of buildings，water，railway and main well lane leaving coal pillar and press coal mining”(Regulations for short). The height of coal seam，the dispensability and compressibility of broken roof gangue，and the size of the broken block of PKS are the main evaluating factors of water-flowing fracture，which are concerned with the critical distance between PKS location and mining seam. The critical value can be generally concluded in the value 7–10 times as the height of coal seam. As the distance from PKS to coal seam is shorter than the critical value，the method based on the Regulations is unreasonable；as the distance from PKS to coal seam is larger than the critical value，the method based on Regulations is available. Therefore，the above research result is the certain explanation to the mechanism of irregular water inrush accident in some coal mines. The achieved results have been employed to conduct the water inrush protection in Bulianta coal mine，Shendong coal field；meanwhile，the significant benefits have been obtained.
