Abstract: In order to reasonably reduce the casing cost of oil and gas wells, the design method of 3Cr steel + 13Cr steel composite casing was used in CO₂ corrosion environment. In this method, the whole wellbore temperature and CO₂ partial pressure profile are used as the basis for casing material selection design, and the whole wellbore casing material is finely designed. The 3Cr steel casing is used in the well section with light corrosion degree, and the corrosion rate of 3Cr steel casing is predicted and the casing strength after corrosion is checked, so as to improve the safety of composite casing material selection and reduce the casing cost. At present, the composite casing design method is widely used in the selection of oil casing materials for drilling and completion operations in the western South China Sea oilfield. Compared with the traditional casing material selection method, the average single well pipe material cost can be reduced by more than 1 million yuan.