Abstract: Changes in weak magnetic signal and metal magnetic memory effects in low carbon A3 steel during torsion are studied.Relationship between signal change and torsion change is studied on a torsion testing equipment.The relationship is presented by recording the vertical component of magnetic intensity on the sample surface and the loading moment of torsion,and the characteristics and reason of these changes are analyzed.The main reason of change in magnetic signal is the directional distribution of the magnetic domain on the stress and the impediment by the dislocation to the domain's turn.A new weak magnetic method of estimating the stress state and micro-damage of component is thus proposed.