Abstract: Soft PLC technology is a new PC-based control technology, which has a strong network communication and data processing capabilities，and it is one of the research hotspots of automation. The welding principle of variable polarity plasma are welding was described, and the design of distributed variable polarity plasma welding control system based on EtherCAT fieldbus and a product of Soft PLC( CoDeSys) of Germany 3S company were introduced. The system can realize the real-time control of the complex space curve trajectory and the plasma welding process, and finally the welding of the complex space curve weld was realized.