Abstract: During the maintenance and replacement of long-distance oil and gas pipelines, it is necessary to demagnetizethe pipelines in order to shorten the replacement cycle and reduce the effect of residual magnetism on the welding qualityof pipelines. In this paper, the reasons for the magnetism of oil and gas pipelines were analyzed, e.g. pipeline inspection, external magnetic field and electric field, pipeline welding and friction. Then, the effect of the magnetism of pipeline on itswelding and service life and other pipeline inspection methods were studied. Finally, with the demagnetization principle ofoil and gas pipeline as the beginning, three pipeline demagnetization methods were proposed to improve the on-line weldingquality of oil and gas pipelines, including DC demagnetization method, AC demagnetization method and permanent magnetdemagnetization method. The research results play a certain role in guiding the selection of demagnetization methods for oiland gas pipelines in engineering practice.