Abstract: In this paper, the mechanism of starting arc pogress is probed and analyzed with the gas discrage theory.According to the characteristic of the electric field pattern which is not uniformed very well in welding gap,the emerged phenomenon on starting arc is explained in the concept of the avalanche,the space charge and the streamer,and the factors affecting the process of starting arc are obtained. The principle is of an importance value for improving the property and developing the new method of the starting arc.