Abstract: In order to study magneto-optical imaging (MOI) law of weld cracks under the excitation of rotating magnetic field, the method of using rotating magneto magnetic field to stimulate weld cracks and acquiring magneto-optical images of cracks by a magneto-optical sensor was used. Theoretical analysis and experimental verification were carried out. The dynamic magneto-optical images under different excitation intensities of rotating magnetic field were obtained. According to the principle of magneto-optical imaging and the theory of rotating magnetic field, the gray value of the obtained data was compared and analyzed. Experimental results show that, under the power frequency excitation of rotating magnetic field, any frame of magneto-optical images will change with the going of excitation time. Taking original three frames as a cycle, each frame of magneto-optical images converts to next frame magneto-optical image in turn. It returns to the initial state after 885 frames of magneto-optical images. The discovery of this law is beneficial to reduce the effective excitation time and improve the effect of non-destructive testing of weld defects.