Abstract: In order to improve the weld formation quality in gas metal arc welding (GMAW) and investigate the effect of low-frequency phase on metal transfer and weld formation, experiments were carried out on low-frequency synchronous and alternating modes respectively under the premise of high-frequency synchronous mode. The metal transfer processes of low-frequency synchronous and alternating modes were photographed using a high-speed camera, and the influence mechanism of metal transfer on the weld pool was analyzed combined with the current change during welding. The result shows that the droplet collision and fusion is reduced, thus the impact of large volume droplet on the weld pool is reduced in the alternating mode, and the splash is reduced. Compared with the synchronous mode, the weld formation is more flat and weld reinforcement variance is smaller in the alternating mode. The fish-scale pattern in the alternating mode is more regular than that of synchronous mode, and the weld formation is optimized.