Contactless magnetohydrodynamic technology has been proposed as a promising approach to optimizing the quality of laser beam welds. However, systematic investigations of the effects of alternating magnetic fields on weld and defect formation are limited. This article systematically investigates the effects of magnetic field parameters on the porosity ratio in laser beam welding (LBW) using both experimental data and a multiphysical model. The magnetic field significantly suppresses porosity. In the reference case, the porosity of the weld is 10.78%; however, the application of a magnetic field can reduce the porosity ratio to below 4%. Results indicate that magnetic flux density contributes more than 85% to porosity suppression, while magnetic field frequency contributes relatively less. Additionally, applying a magnetic field increases weld width and reduces penetration depth. This phenomenon is attributed to the enhanced lateral flow of the molten pool on the upper surface in the presence of a magnetic field. Meanwhile, the Joule heating induced by the magnetic field is negligible and has little influence on the weld pool volume. Thus, when the weld pool volume remains constant with the same laser power and welding velocity, the intensified lateral flow of surface molten metal results in a wider weld and shallower penetration. While the change in flow behavior improves surface weld formation, it also promotes undercut defects.