Abstract: In the offshore oil and gas development, casing recovery technology is mainly used in sidetracking and abandoned well shaft treatment. Sidetracking technology is gradually increasingly used in the oil and gas fields of the East China Sea, mainly to improve the production and recovery, and realize the reuse of well slots. But in the casing recovery technology, new technical problems are also emerging, such as the difficulty in recovering large size casing, the uncertainties in the annulus cement quality between borehole and casing and in the annulus medium distribution, as well as the deviation of the casing from the center position. In the East China Sea, the recovery of a large size casing is successfully performed. During the recovery process of the 339.7 mm casing of A10 well, some techniques are used, including the isolation scanner (IBC) logging technology, the optimization of cutting position, and the application of fishing, casing milling, milling and other processes. The successful recovery could provide conditions for subsequent sidetrack drilling. These technologies can also be used for similar operations.