Abstract: Due to the low degree of automation and low production efficiency for back welding, the pulse MAG-TIG twin-arc tandem root welding process on thick plate with 24 mm was carried out, welding process and weld seam forming mechanism were investigated. The results indicated that TIG arc had regulation effect on MAG arc heat distribution when arc discharged. Combined with arc plasma behaviors and molten pool state, a result could be found that, when the bead back formation was optimum, TIG arc accelerated the weld pool to flow backwards. Thus the quantity of liquid metal was reduced at the bottom of the molten pool. At this condition, molten pool with balance stress and steady state was obtained, and a sound, uniform and stable surface appearance back weld was acquired.