Abstract: The static tensile test of Q235 steel sample was carried out in different magnetic field environment, the normal magnetic field of the surface of the sample under stress concentration was collected up in-situ by the TSC-1M-4 tester and variation of the magnetic field in static tensile was analyzed. The result showed that, noise interference could be reduced effectively by applying an external magnetic field, thus improving the detection result. Stress-induced magnetization reversal phenomenon occurred in elastic stage, and kept the reversal in the plastic stage, then the magnetic field crossed zero and increased in the opposite direction. The correlation between magnetic gradient K and stress in the external magnetic field environment was discussed.