Abstract: In order to study the characteristics of magnetic-optical images of weld defects excited by magnetic field, low carbon steel of laser welding was used as test object and the welding defects were excited by constant magnetic field and 50Hz alternating magnetic field. Real-time magnetic field distribution of welding defect area was obtained by magnetic-optical imaging sensor. Through theoretical analysis and experimental verification, the magneto-optic images of welding defects of low carbon steel (Q235) of thickness of 1mm, 2mm and 3mm with constant magnetic field and alternating magnetic field were obtained, and then, were compared with COMSOL simulation results. The weighted average image fusion technique was used to fuse the magnetic-optical images of welding defects in the alternating magnetic field. The results show that, compared with the excitation of constant magnetic field, the welding defect information obtained by the alternating excitation is more accurate, fast and complete, and avoids the omission of welding defect information effectively. This study provides the basis for improving the detection efficiency of welding defects.