The southwest edge of Erdos basin is an extreme water shortage area. There are many water wells in cretaceous water-bearing formation, but water quality degradation and water quantity decreasing are common phenomenon, which is mainly caused by the serious unreasonable well structure. Based on the analysis on the causes of water well quality degradation, the substantial issues was found that water quantity decreasing was caused by well structure deformation because of unreasonable water stoppage method and diameter shrinkage in mudstone. The scientific and reasonable well-forming scheme was put forward to solve the water well quality issue and prolong the water well service life.