As a kind of efficient welding method, triple-wire welding is taken seriously, and the arc shape and arcing characteristics are important factors in reflecting the stability of welding process and determining the weld quality and formation. Triple-wire metal inert-gas(MIG) welding system has been established in the test. In the welding process, the waveforms of welding current and arc voltage are acquired through the MultDaq signal acquisition system while the arc behavior and metal transfer process are monitored by high-speed photography system synchronously. Then the whole welding process can be monitored in real time, the mechanism of alternate arcing in different metal transfer modes and the influence of arc voltage on alternate frequency are studied. The results of experiments show that when the welding power source can only supply less energy, the current distribution on three wires connected in parallel presents the rule of “shift”, which results in alternate arcing among wires. Moreover, current distribution on three wires is decided by the location of wires in the arc space. As the increases of the arc voltage, the alternate frequency decreases. When the arc voltage is up to 34V, the three wires start arcing simultaneously, no alternate arcing, and alternate frequency becomes 0 Hz.