Abstract: Using MAG welding, select appropriate welding process parameters to conduct welding process tests on LGQ355B steel and LGQ345B steels, and compare the mechanical properties of the welded joints of the two steels under the same welding conditions. The results indicate that: Compared with LGQ345B steel, LGQ355B steel has higher tensile strength and yield strength, with an average increase of 30MPa, while LGQ345B steel has better plasticity and impact toughness, and the longitudinal impact performance of the two base materials is significantly better than the transverse impact performance.The mechanical properties of the two types of steel welded joints meet the standard requirements, mainly related to the characteristics of the base material itself and the selected welding wire. Due to the lower strength of the LGQ345B steel base material compared to LGQ355B steel, its welded joints exhibit high-strength matching, and the center impact toughness of the weld seam is better than that of LGQ355B steel. The overall impact toughness of the weld seam center of the two types of steel welded joints is better than that of the base material.Through research, it has been found that in the manufacturing process of daily structural components, the difference in welding process and joint performance between Q355B steel under the new standard and Q345B steel under the old standard is not very significant. The welding process does not need to be treated differently and can be replaced normally. At the same time, it provides theoretical guidance for the welding of Q355B steel under the new standard.