The increase of the size of coal seam mining will increase the influence range of mining surrounding rock in mining, especially under the condition of strong aquifer in coal floor, the influence of surface length effect on the stability of floor is of great significance. The North China-type mining areas are affected by the high ash and austral ash aquifers at the bottom of the Carboniferous Permian coal system. The risk of water inrush from the floor is closely related to the sufficient degree of mining. Through the numerical simulation of water-rock coupling, the characteristics of rock pressure in the aquifer under different surface length conditions were analyzed. The fish program was used to adjust the rock into the strain-softening model to study the influence of the surface length effect. The degree of damage of the aquifuge and the shape of the damaged zone were studied. The research results provided reference for the prevention measures for safe mining of pressurized water.