Abstract: The cracking reason of 177.8 mm × 11.51 mm P110 oil casing after the hydraulic test was analyzed by means of chemical composition analysis, fracture analysis and microstructure examination.The results showed that the cracking reason was the casing tube organization distributed large inclusions, and its existence undermined the continuity of the materials organization and weakened transverse tensile strength, the stress concentration under the conditions of local stress exceeded the actual tensile strength.