Abstract: To investigate the influence of MgO on the welding process performance and slag properties of gas-shielded flux-cored wires for low-alloy steel used in storage tanks, the influence mechanisms of the wires without MgO and with 6% MgO on the weld formation performance were comparatively analyzed. This analysis was conducted using methods such as the drop-ball impact test, direct-reading spectroscopy, laser confocal microscopy, scanning electron microscopy, X-ray diffraction, and energy-dispersive spectroscopy. The results indicate that the slag detachment rates of both wires reach 100%; however, after the addition of MgO, the surface of the weld metal is smoother, presenting a uniform fish-scale morphology. The average value of slag thickness increases from 220 μm to 557 μm, and the variance decreases from 5 246 μm² to 235 μm². Moreover, the slag still provides good coverage over the weld under high welding currents and voltages, which mitigates the adverse effects of the slag on weld formation. When MgO is not added, the inner surface of the slag is mainly composed of massive and rod-bundle-shaped coarse crystals mainly containing Mg, Ti, and O. The presence of these coarse crystals induces a large internal stress within the slag, which causes the slag to fall off in small segments during detachment and makes it prone to fragmentation. After the addition of MgO, the inner surface of the slag is mainly composed of continuously distributed amorphous substances mainly containing Al, Si, Mg, and O. The surface energy between these amorphous substances and the weld metal is relatively small, which blocks the interaction between the weld metal and the slag and promotes the overall detachment of the slag, thereby improving the weld formation performance.