The effects of weld current, voltage, frequency and duty cycle on weld forming performance were investigated in SAW (Submerged Arc Welding) with square wave AC mode. The results showed that the penetration increased with the decrease of frequency and increase of duty cycle, but weld width showed the opposite change tendency. Furthermore, some experiments were implemented to analyze the effect of wire extension length and arc voltage on bead appearance. All of these experimental data could provide proper parameters for the lead arc and the trail arc in tandem SAW using square wave AC and also testified square wave AC was preferred over DC and traditional 50Hz AC. The square wave AC power source could be applied widely in tandem SAW.