Abstract: In this study, the defects in the wall of petroleum casing steel pipe were investigated. The morphology and composition of inclusions in the defects of the steel pipe were analyzed using scanning electron microscopy–energy-dispersive X-ray spectroscopy. The thermodynamic calculation of the Ca‒Al equilibrium phase diagram of molten steel in tundish and the changes of the ladle slag phase composition with cooling temperature was performed using FactSage8.0. The results show that the longitudinal surface of the defect is mainly composed of shallow and deep stripes. A large number of MgO·Al₂O₃ inclusions containing a small amount of Mn is detected at shallow stripes, and a large number of inclusions, such as Al₂O₃, MgO·Al₂O₃, and CaO·Al₂O₃·SiO₂ are detected at deep stripes. The three main types of inclusions in the cross-section of the defect zone are CaO·Al₂O₃·SiO₂, CaO·Al₂O₃·MgO, and CaO·Al₂O₃·MgO·SiO₂. According to the analysis results of inclusions in the cross-section and the calculation results of the phase transformation of slag droplets during solidification and cooling, the formation mechanism of the defects in the wall of steel pipe can be speculated as follows: (1) At the end of pouring, the ladle slag in molten steel in the ladle enters the tundish. Further, the slag droplets adsorb the fine xAl₂O₃·yCaO or Al₂O₃ inclusions with high Al₂O₃ content in molten steel, increasing the Al₂O₃ and CaO contents in the slag droplets. (2) Ladle slag in molten steel is subjected to strong stirring in Ar gas in the vacuum degassing (VD) refining process. Moreover, the slag droplets adsorb the fine Al₂O₃ inclusions in molten steel, increasing the Al₂O₃ content in the slag droplets. During solidification and cooling, the slag droplets formed in the two aforementioned forms of inclusions are transformed into three types of inclusions: CaO·Al₂O₃·SiO₂, CaO·Al₂O₃·MgO, and CaO·Al₂O₃·SiO₂·MgO. In the process of round billet piercing deformation, under the action of longitudinal tensile stress and transverse shear stress, the large slag droplets involved extend along the longitudinal cross-section and finally form defects in the wall of the steel pipe.