Abstract: Starting from analysing the efficiency of constant current analogue-type transistor arc welding power source,the authors found that it is necessary and rational to develop a switch-type transistor arc welding power source. After mathematical analysis of the work process of main circuit, the function relationship of the regulation sensitivity to the ratio of pulse width was abtained. In this paper, the problems of choosing and protecting large power transistors have been analyzed theoretically in detail. Especially, a new idea of originality was put forward by using series resistance in emitter to avoid secondary voltage breakdown. The paper also provides the technical data of the 120A switch-type arc welding power source and the experimental results of welding a circular butt joint of stainless steel thin-wall shell with TIG.