Abstract: With consideration of the service conditions and actual mechanical loads of the casing string working in the deep and ultra-deep wells,the advanced casing string design codes as made abroad are analyzed.Also studied as the focus are the problems like casing strength reduction as caused by wear of the casing string which occurs during the process of drilling-in operation when building a deep or ultra-deep well,or during the production process.Based on the even-wear model and the eccentric cylinder model,the mechanical properties of the worn casing are analyzed,and the accuracies of the two computation methods are compared with relevant FEA software.As a result,it is identified that the calculation result from the eccentric cylinder model is more practical,being capable of improving the strength design quality of the casing string.