The formed part manufactured by rapid prototyping of arc welding has the advantages of good mechanical properties,high density and broad development prospects. In order to control the dimension of formed part and improve the limits of parameters,a double wire arc welding system is designed. The design of this system hardware is mainly introduced. A set of thin wire feeder is equipped with the conventional welding system,and an integrated double wire welding gun is designed to meet the requirement of system integration. Siemens S7-200 PLC is used as the control part,at the same time,a human-computer interface is designed to input and display parameters for the convenience of human-computer interaction. Experiments show that both coarse and thin wires can be welded well. Furthermore,the system can switch smoothly with continuous arc in welding process.