Abstract: Simulation and experiment of magneto-optical imaging non-destructive testing of weld under alternating electromagnetic field were carried out. The weld crack magnetization under the alternating magnetic field of carbon steel (Q235) was studied. Based on a simulation model of alternating magnetic field excitation, the magnetic field density (magnetic flux density) curve distribution of the weld crack with 0.03, 0.05, 0.07 and 0.1 mm gaps were analyzed. Simulation results were verified by the magneto-optical imaging nondestructive testing system. Experimental results show that the weld cracks of different gaps have the same shape of the flux density mode curve, and there is significant difference between N pole and S pole. The amplitude of the flux density curve and the transition area will decrease as the crack gap decreases.