Abstract: The high-speed video images of short-circuiting transfer process in CO2 arc welding are processed by digital image processing technology, obtaining the equivalent radius size of contact droplet and residual droplet. The relationship between the transfer frequency and equivalent radius and the stability of short-circuiting transfer process is analyzed. The result shows that the welding parameter that corresponds to the largest transfer frequency is not the optimal parameter. Further, on a small scale the equivalent radius distribution of residual droplet is discrete, and the average value is minimum under the condition of optimal welding parameter. Meanwhile, the average equivalent radius of contact droplet is proportional to the volume of the transfer droplet. The smaller the average value of contact droplet size, the more stable of the transfer process.