Abstract: The backing weld of high strength steel with Y groove was achieved by means of bypass-current wire-heating PAW, which had advantages of high melting efficiency of wire-heating, strong penetration ability of PAW, controllable heat input of bypass-current arc. One-side welding with back formation of medium and heavy plate was realized by this method. The results indicated that the plate was full penetrated and the weld formation was sound without welding defect. The microstructure of the back weld was refined after reheating effect in the subsequent multi-layer and multi-pass welding. The hardness of the weld zone was slightly higher than that of the base metal, and the hardness of heat affected zone was lowest. The hardness of the weld zone decreased gradually from root bead to top bead. Tensile test results showed that ductile fracture occurred in the base metal and the characteristic of fracture appearance was dimple.