Abstract: Two qualitative models were established by referring to the method to explain skin effect of alternating current magnetic field. Those models could deduce and interpret the phenomena that longitudinal discontinuities defects in the pipe inner surface which were difficult to disclose when longitudinal magnetization using alternating current coils was carried out, and there was a big difference of sensitivity in inner and outer surface when workpieces were magnetized using central conductor through alternating current. Conclusion was drawn that, when alternating current was passing through the coil, magnetic fields in tube-shaped workpieces were mainly collected in outer surface and that in inner surface were weakened or even counteracted. On the other hand, when the workpiece was magnetized by central conductor through alternating current, magnetic fields were in both sides, but the intensity was different, which appeared not to follow empirical formula. This conclusion could explain experiment phenomenon and help to select magnetization methods to detect tube-shaped workpieces.