In this paper, the diameter of 1.2mm and 1.6mm welding wires were used to carry out high-frequency pulse MAG butt welding process test on S355J2W+N weathering steel. The analysis of the microstructure characteristics of the welded joints under the two working conditions shows that there is little difference and both are ideal. There were more acculate ferrite (AF), less pearlite (P) and bainite (BG), and no larger Weisserite structure and less ferrite side-plate (FSP) structure. The weld is well formed, weld defects such as gas pores and cracks are not found. The impact test showed that the impact energy Akv (-40 °C) of the welded joint at -40 °C was higher than the specified minimum impact energy of 27 J. Compared with the 1.2mm welding wire, the impact energy of the 1.6mm welding wire is lower. Tensile test results show that the fracture occurs at the base metal position. and the fracture height is uneven, forming an angle of 45° with the tensile stress direction, which is a ductile fracture. When the bending angle of the butt joint samples reaches 180°, no cracks are both found on the tensile surface, and the plastic index of the welded joint samples is qualified and has good bending performance.