Abstract: The 2 m long and 50 mm thick plate was welded by the double-sided arc welding process. The welding productivity of the thick plate by double-sided arc welding process was 1.4 times more than that of the troditional single MAG arc welding, and the mechanical properties of welded joint could meet the requirements. The metallurgical structures in the HAZ by double-sided TIG arc backing welding were the lath martensite, and those in the welding seam were composed of the acicular ferrite and the lath martensite.