Abstract: In certain special positions of oil and gas pipelines, the magnetic flux leakage detector can hardly be used to detect signals related to magnetic flux leakage. Since pipelines may be magnetized by magnetic flux leakage detection magnetizer to magnetic saturation with residual magnetism produced, coil excitation, or the residual magnetism can be deployed to detect pipeline defect. This paper reviews residual magnetism detection involving magnetizer, principles related to residual magnetism effect and residual magnetism detection. Within relevant tests, coil excitation is used to magnetize pipelines to magnetic saturation to generate residual magnetism. Then, magnetic-sensitive elements are used for external inspections by using signals of residual magnetism. Research results show that radial depths and axial width of pipeline defect are in linear relationship with signals of residual magnetism. Accordingly, such signals can be used to detect pipeline defects accurately. In addition, residual magnetism effect has been proved to be useful for detection of pipeline defects.