<p>This paper introduces a magnetic-field-assisted (MFA) variable polarity gas metal arc welding (VP-GMAW) system. We developed a novel excitation power supply and magnetic field generation device suitable for VP-GMAW. A finite element model of the excitation device was established to analyze the three-dimensional spatial distribution of the magnetic field. Furthermore, we established a mathematical model describing the motion of charged particles. Simulation results show that the longitudinal alternating magnetic field induces a near-helical motion of particles on the arc surface. Macroscopically, this appears as a rotational constriction at the top and an expansion at the bottom, presenting a bell-shaped arc morphology. These results concur well with the arc shape captured during the ignition phase via high-speed imaging. Experimental results indicate that the external magnetic field (EMF) enhances the wettability on the surface of the base metal, increases the weld width, reduces the weld reinforcement, and causes the formation of uniform fish-scale patterns. Furthermore, the formation of intermetallic compounds (IMCs) is effectively suppressed, presenting a considerable improvement in the tensile performance of the joint. Specifically, the thickness of IMC layer is reduced by up to 46%, and the tensile strength reaches as high as 84.4% of that of the base metal. This work not only provides experimental evidence and theoretical guidance for the application of MFA-GMAW, but also establishes a particle-level modeling framework that lays the foundation for real-time magnetic field control in future research.</p>