Objective To study the causes and mechanism of blackening defect seriously affecting the phosphating quality of cold-rolled steel sheet. Methods Blackening defect was systematically analyzed by methods of metalloscope, EDS, XPS, GDS, XRD, etc. , and the formation mechanism was discussed. Results The C element content of the surface of defect steel sheet was nearly 17 times that of normal steel sheet. The C element of the surface of defect steel sheet mainly existed in the form of graphite. Conclusion Blackening defect was caused by massive accumulation of C element in the form of graphite on the surface layer of steel strip, which was closely related with the chemical composition, surface status, and production process such as annealing of the steel sheet.