Abstract: In order to master the wear depth and residual strength of downhole casing,the author in this paper,based on the data obtained from the drilling history of oilfields in China,uses the wear efficiency model to analyze the depth of crescent wear of casing,and presents the approach and formula for calculating the wear depth of downhole casing,laying a basis for subsequent drilling and production test.With the bipolar coordinate approach in elasticity mechanics,this paper converts the calculation of off-worn casing strength featuring in two non concentric boundaries into an axisymmetric concentric issue,which is easier to be analyzed.The circular stress expression of wearing casing under internal and external pressures is obtained after the deductions.Judged by whether the stresses of inner wall of the thinnest part of wearing casing reach the ultimate yield of pipes,the residual collapse strength and residual strength resisting inner pressure is obtained.The methods mentioned above have been applied to such important exploration wells as Yangta-8 and Mu-7 with depth more than 5000 m,laying a solid basis for safety production test.