Abstract: Mining in Huainan old mineral area and Xinji mineral area is threaten by karstic water of coal seat. They mine by limiting pressure and ascertain a water gushing coefficient. However, we analyzed a great deal of data in Huainan detailedly and have done experiment which imitated scaleboard breakage in mining.And then we found that water gushing coefficient was conservative. The key point is that the consideration of water-resisting layer was conservative.They have considered soft rock under the coal as the water-resisting layer. We considered that soft rock layer and hard rock layer alternating can be considered as relative water-resisting layer. Experimen phenomena have indicated that soft rock layer and hard rock layer alternating can protect each other,and reduce the depth of deformation destruction.