In this paper, a new welding experiment is studied by applying an additional longitudinal electromagnetic field to CO2 welding process (abbr. LEM-CO2 welding).The characteristics of droplet transfer, macrostructure and microstructure are compared between LEM- CO2 welding and general CO2 welding on Q235 low carbon steel thick plate joint. The research results shows that, an additional longitudinal magnetic field can have a significant effect on properties of the droplet transfer in CO2 welding, the frequency and stability of the droplet transfer in LEM-CO2 welding are improved. The grains of welding seam are refined and welding joints has a higher quality. So the additional longitudinal magnetic field is a very simple and effective method to improve the properties of CO2 welding thick plate joint.