Abstract: The J55 casing with a specification of 177.8 mm × 6.91 mm (outer diameter × wall thickness, the same below) was used to replace the J55 casing with a specification of 177.8 mm × 8.05 mm for cementing during the exploitation of a water well. The J55 casing with a specification of 177.8 mm × 6.91 mm fractured during the well drilling process. The macroscopic observation, chemical composition analysis, mechanical property test, metallographic examination, scanning electron microscopy analysis and other methods were used to analyze the causes of casing fracture. The results show that the matrix structure of the fractured J55 casing sample was pearlite plus reticular ferrite. There was a serious mixed crystal phenomenon near the fracture, and there were small cracks on the outer surface of the steel pipe at the fracture. In the process of over-design and use, the tension and torsion force of the steel pipe reached the limit value that the steel pipe section could actually bear. At the same time, the small cracks on the outer surface of the steel pipe became the source of crack initiation. Under the action of external force, the cracks propagated along the coarse grains to the inner surface, which eventually led to the fracture of the casing.