Abstract: Magnetical-optical (MO) sensor could be used to detect and track microgap weld (≤0. 1 mm) for laser welding. In the case of using a DC magnetic field to magnetize the weld of the weldment, the magnetic charge theory was used to set up leakage magnetic field distribution model for nearby microgap weld surface. And then, this magnetic field distribution feature was converted to microgap weld MO image by MO sensor. Through the comparative analysis and study of the magnetic field distribution characteristics near the surface of the weld and the corresponding characteristics of the transitional zone in the MO image of the weld, the transitional zone center of the weld in the MO image of the microgap weld was determined to correspond to the actual weld center. The research result showed that the model could effectively explain MO weld information, which provided a reasonable theoretical basis for weld tracking and identification by MO image of the weld.