Abstract: In order to identify and determine effectively the girth weld (GW) defect of pipelines based on magnetic flux leakage (MFL) in-line inspection (ILI), the GW features of abnormal signals and their influential factors were analyzed. Then, the effects of pipeline magnetization degree, sensor liftoff, GW remaining height, and shape, position and opening orientation of GW defect on the defect magnetic flux leakage field were investigated systematically by carrying out finite-element simulation analysis and in-line inspection traction test on MFL signals. As a result, the relationship between GW defects and MFL signal characteristics is clarified. And finally, the GW defect classification method based on MFL ILI signal was proposed. And accordingly, GW defects were classified into four types, and their detection ratio and identification precision ratio based on MFL ILI were also provided. The actual excavation results verify the identification and classification accuracy of this method. It provides the foundation for the industrial application of GW defect identification and determination technology based on MFL ILI.