Abstract: Two types of approximate theories for Betzs ring in magnetic particle testing were briefly introduced, which were the approaching of equivalent magnetic dipole strengths and equivalent hole depths as well as the approaching of magnetic dipole chains. From the later, the characteristic curves between the number of small hole and the critical applied magnetizing current, by which the magnetic particle pattern of the buried hole were revealed on the ring circumference, were calculated and plotted. The tendency of one of these characteristics is identical with the experimental curves. And it was also proved that for the magnetizing electric current there is no need to reach saturation to reveal all twelve holes of Betzs ring.