Abstract: An experimental system to photograph molten droplet images by using a high-speed charge coupled device camera and to record welding current and arc voltage in phase was established.After the definition of molten droplet size in short-circuit transfer mode was given and the analysis system for molten droplet size and arc signals based on MATLAB platform was introduced,the characteristic of molten droplet size in short-circuit CO₂ arc welding using the power supply with constant voltage output,the relationship between the molten droplet size and the welding process behavior were investigated.The experimental results show that the molten droplet size presents normal distribution between 1 and 2 times of wire electrode diameter.The excessive large or small size in molten droplet are all unfavorable to the stability of welding process in short-circuit transfer mode.According to the formation and transfer course of molten droplet,the main influence factors to the molten droplet size are analyzed and the approaches to control the droplet size are proposed.Namely,the random characteristic of the residual liquid metal quantity at the end of wire electrode after the completion of short-circuit transfer and the arc energy induces the uncertainty of molten droplet size.(making effective control on them will increase the accordance of molten droplet size and short-circuit transfer procedure,and improve the process behavior and welding quality of short-circuit CO₂ arc welding further.)