Abstract: The tensile samples were made of Q345R steel which was used commonly in stress vessel. The variation of low-alloy steels surface magnetic field intensity HP, magnetic field gradient K and mmax according to tensile stress and plastic strain were studied during online and offline tensile testing, so as to examine whether these parameters could be utilized to determine their deformation state. The results suggested that the deformation state could be identified by the shape of HP signals and the magnitude of mmax during online inspection, while the intensity of HP signals and Kmax could be used during offline inspection.