To investigate the signal characteristics of the specimen under different tensile loads, the normal component of surface spontaneous magnetic signal is measured on-line by the static tensile test of the Q345B steel plate specimen. The effects of stress on the normal component Hp(y) at the fracture position and characteristic parameters are analyzed. The quantitative correlation between the stress and magnetic field gradient index is obtained. The results show that Hp(y) signal reverses to the magnetic field in elastic stage when the stress is greater than 308 MPa. The maximum value of maximum gradient Kmax can be used to determine whether the specimen enters the plastic stage. The arithmetic mean of maximum gradient Kmax , namely Kavg max which varies with stress agrees with the elastic, plastic and failure stage of the force-displacement curve. Both Kavg max and Hp(y) at the fracture position can reflect the degree of damage, and gradient indexξcan evaluate the stress state of the specimen. Analyzed results reveal that Jiles-Atherton model can well explain the reversal phenomenon of magnetic signal. The model of inductive magnetic field can qualitatively explain the reason of the variation of magnetic field at the fracture position, which is consistent with the experimental results. The study suggests an effective stress evaluation method based on metal magnetic memory technique.