Abstract: A pulsed gas metal arc welding (P-GMAW) power source with stepped external characteristic was developed using a BUCK main circuit topology. The external characteristic was set automatically to be constant current, constant voltage, short circuit or open circuit according to the arc working point. Experiments were carried out on Q235 mild steel to investigate the influence of different output characteristics on P-GMAW process. The results indicated that the arc was lack of stability when applying constant voltage characteristic at background time, making it easy to occur arc extinction or short-circuiting. When constant current characteristic was applied during background time, on the other hand, arc extinguishment and spatters were reduced, but short-circuiting still existed. However, adopting stepped characteristic could effectively decline or even eliminate short-circuiting and arc extinguishment phenomena, resulting in pleasant stability. Using stepped characteristic at background time and constant voltage characteristic at peak time, the arc had strong interference resistant capability, and the welding process is most stable among the experiments resulting in a uniform weld appearance without any spatter and arc interruption.