Metal magnetic memory (MMM) method is a novel, passive magnetic method for inspecting mechanical degradation of ferromagnetic components. In this paper, the correlation between large deformation and the deformation-induced magnetic field was experimentally investigated for the X80 steel. Various tensile plastic deformations are introduced into test specimens with different initial defects. The surface strain distributions and normal natural magnetic fields component are measured after the tensile testing. The results show that an abnormal wave occurred in the stress concentration zone and its height value was sensitive to deformation levels, the relationship between the intensity of damage-induced magnetization and the maximum plastic strain follows a simple formula proposed in this paper. The magnetic field signal depend on the deformation in accordance with the simulation results based on the mechanical-magnetic coupling FEM model. Different magnetic signal characteristics in the plastic region indicate that the magnetic memory technique can identify macro yielding and early damage.