Abstract: Triple-electrode MIG welding system was established in this paper. The welding current and arc voltage were acquired through a signal acquisition system, and the arc behavior and metal transfer process were monitored with high-speed photography system synchronously. Three basic arcing modes were observed by changing preset voltage, which could be used to analyze the establishment of mutual conducting channel. The experimental results show that the basic arcing forms were single-wire arcing, twin-wire arcing, and triple-wire arcing. The establishment of mutual conducting channel can be divided into three phases. First, the conducting channel was established between three electrodes and base metal, respectively. Second, in order to establish the mutual conducting channel, three conducting channels broke the respective channel under electromagnetic attractive force. Last, a new conducting channel adjusted to steady state. Moreover, the mutual conducting channel could help to stable arc burning.