A new method and its principle for sensing droplet transfer in GMA W of steel and aluminum alloy have been researched in this paper. A practical arc light sensing and controlling system has been developed. The reliability of the arc light characteristic signal that indicates droplet detachment and the control accuracy of the system have been verified using high-speed photography. Based on the mathematical model set up in the paper, the mechanism of the signal and relative phenomena were analyzed. The parameters which influence arc light radiation were given and the limitation of this sensing method was discussed .