Abstract: Qianan area of Jilin oilfield has entered mid-late stage of development and casing damage in water injection wells is serious,which significantly affects the waterflooding effect.Taking Daqingzi oilfield in Qianan as a study area,the characteristics of casing damage and statistical distribution of casing damage have been analyzed,experimental study on rock mechanics for casing damage intervals has been carried out,and the numerical simulation is conducted on in-situ stress and other factors.By conducting this systematic investigation,the major factors resulting in the casing damage are determined as following:the softening of mudstone after water absorbing leads to the elliptical distribution of nonuniform in-situ stress,which significantly reduces the collapse resistance of casing.Based on these new findings,the casing damage prevention strategy is proposed,and the optimized design for casing in serious casing damage intervals is developed.