Abstract: Since the presence of CO₂, the droplet control to MAG welding has always been a difficult problem. Based on preliminary studies, ultrasonic standing wave field, as a new method of droplet regulation, was introduced into MAG welding in this paper. In different conditions of shielding gas composition, the effect of ultrasound on droplet transfer was also studied. Under the conditions of low CO₂ content, ultrasonic effect was still very significant. But with the increase of CO₂ content, especially more than 20%, in short transition, the compression effect of ultrasound-arc tended to a constant and minimum. In droplet transition, this electromagnetic force gradually increased and eventually offset the downward ultrasound radiation force. Finally,the conclusions of the above analysis was verified by pure CO₂ ultrasonic-MAG welding experiment.