Abstract: In view of the operating characteristics of pressure pipelines in the petrochemical industry, research on magnetic flux leakage outside the pipeline is carried out according to the principle of magnetic flux leakage detection. The finite element analysis software was used to establish a magnetic flux leakage detection model for the double-excitation structure pipeline, and the spatial distribution characteristics of the magnetic flux leakage of the hemispherical defect were obtained. The influence of the defect volume, pipe wall thickness, the value of the air gap, the opening angle of the two magnetized structures and other factors on the magnetic flux leakage signal of the defect were studied, and experimental research was correspondingly conducted. Both test results and simulation results show that the magnetic flux density of the defect leakage field increases with the increase of the defect volume, and decreases with the increase of the pipe wall thickness and the air gap; the axial component of the defect leakage field magnetic flux density decreases with the increase of the opening angle. The magnetic leakage detector can effectively identify hemispherical defects, and can quantify and locate the defects.