Abstract: Generating and welding dual use set consists of generator, welding inverter, automatic control system, and internal-combustion engine controlled by speed regulator. Each winding of the generator consists of four parts. When the parts were connected in series by shifting switch, the set can serve as general generator, when they are in parallel, the set serves lower AC voltage to welding inverter. Thus high frequence transformer is not needed anymore in welding inverter. Analysis and calculation show that harmonic waves can lead to obvious difference between effective value and mean value of voltage, and the output voltage rectified by diode bridge varies with load. So the controller for excitation system is designed to collect the voltage rectified by diode bridge as its input. Hence the dual use set has proved to be low cost, multi functional, reliable, stable, easy to maintain and have a good dynamic characterisics, which also provides the foundations for more advanced control.