Abstract: In order to better undertake field detection and quantitative analysis of defects, it is urgent to explore the effect of the type and depth of defects on the leakage magnetic field signal. According to the standard of NB/T 47013.12-2015, the pipeline calibration specimen and the contrast specimen and the crack specimen were prepared. The diameter-variable magnetic flux leakage testing device outside pipeline was carried out on the pipe diameter of 38.1, 50.8,76.2,101.6 mm, which were small defects such as pitting and crack. The experimental results show that the leakage magnetic field signal of the outer wall defect is stronger than that of the inner wall; the leakage magnetic field signal of the inner and outer wall is enhanced with the increase of the defect depth; the leakage magnetic field signal of the groove defect is stronger than that of the spherical defect.