Abstract: The short-circuit transformation is always adopted in CO₂ arc welding process,which has disadvantages of high spatter and bad welding formation.Basing on the study of short-circuit transformation in CO₂ arc welding,the paper presented the relation between the parameters in each phase in surface tension tranformation(STT) and the wire feed.The method to test the welding voltage and the state of liquid bridge is also presented in this paper.The results show that the droplet is out of control before liquid bridge breading,which increases the unstablity of STT.With the variety of wire feed speed,the interval of each phase of droplet transformation has a great influence on the breaking state of liquid bridges.According to the requirement to the welding power in STT,two designs,invert-loop control mode and output control mode,have been pre-sented.The limitation of the invert-loop control mode in STT control is also put forward.The result shows that by adjusting the current in each phase can improve the stability of the transformation in STT.The project has been validated by experiment.