Abstract: Abstract In order to study the relationship between defect parameters and MFL (Magnetic flux leakage) signal, COMSOL multiphysics simulation software was used to simulate a variety of defects with different lengths, depths and cross-section shapes. Several groups of defects were constructed by selecting different defect parameters, the simulation model of defect leakage magnetic field was established, and the signal waveform generated by different defects excitation was obtained. The least square curve fitting method was used to fit the simulation data and the defect size. The simulation and experimental results showed that the peak value of the axial magnetic flux component was directly proportional to the defect depth and the two-dimensional edge contour area of the defect, and the peak distance of the radial magnetic flux component was also directly proportional to the defect length. The results provided a basis for MFL signal evaluation of pipeline defect size.