Abstract: A new ZX5-400D type multi-purpose SCR arc welding power source with microcomputer controller has been developed. Its hardware and software designs are introduced in this paper. The new welder can provide multiple output features, such as D.C. pulse clirrcnt, D.C square wave current, adjustable static characteristics and so on , only by operating defferent parts of its control program. The new type welding machine is succcsful in applying the advantages of the microcomputer control and shows an effective way of developing and rebuilding the conventional welding power sources.