Abstract: In order to study the effect of defects on the surface magnetic field of specimen, a series of stepwise loading tensile tests were carried out on 30Cr alloy steel. Defects were artificially prefabricated in advance on the surface of specimens. The magnetic field signal was collected by a TSC-1M-4 magnetic detector and the characteristics of magnetic field signals were studied. It shows that the tangential magnetic field is more sensitive to the local yield of specimen. The surface magnetic memory signal changes significantly at the edge of defects. Thus, it is an feasible method to evaluate the state of stress concentration by surface magnetic memory signal.