Abstract: The defects in the wall of one petroleum casing steel pipe are analyzed with SEM-EDS and FactSage8.0. The results show that the longitudinal surface of the defect is mainly composed of shallow stripes and deep stripes. There are a large number of MgO·Al2O3 inclusions at the shallow stripes, and a large number of Al2O3、MgO·Al2O3、CaO·Al2O3·SiO2 and other inclusions at the deep stripes. The inclusions on the defect cross section are mainly three types of inclusions of CaO·Al2O3·SiO2、CaO·Al2O3·MgO and CaO·Al2O3·MgO·SiO2. The formation mechanism of defects in steel pipe wall is as follows: ①At the end of pouring, the ladle slag involved in the molten steel in the ladle enters into the tundish. Then the slag droplets adsorb the fine xAl2O3·yCaO or Al2O3 inclusions with high Al2O3 content in the molten steel, resulting in the increase of Al2O3 content in the slag drop; ②Ladle slag is involved in the molten steel in ladle under the large stirring of Ar gas in VD refine process. Then the slag droplets adsorb the fine Al2O3 inclusions in the molten steel, resulting in the increase of Al2O3 content in the slag drop. During the solidification and cooling, the slag droplets formed in the above two forms are transformed into three types of inclusions: CaO·Al2O3·SiO2, CaO·Al2O3·MgO, 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 and cross section, and finally form defects in the steel pipe wall.