Abstract: In this paper, Q345R carbon steel was selected as the research object, different proportions of Ar+CO₂ mixed gas was used to design five sets of experiments. The impact of gas composition on the performance and microstructure of the welded joints was analyzed through metallographic observation, hardness testing, tensile testing, and low-temperature impact testing. The experimental results indicate that there is a trend towards uniformization and refinement of the microstructure in the weld seam zone with the increasing of argon gas content. The welded joint with φ(Ar) 80%+φ(CO₂) 20% gas protection has the best performance, and the hardness of weld seam zone, heat affected zone and base metal zone are 188HV10, 199 HV10 and 204 HV10, respectively, and the tensile strength is 518.5Mpa. The average impact energy of weld seam zone and fusion zone at 0℃ is 110 J and 69 J respectively, and that of weld seam zone and fusion zone at -20℃ is 100 J and 49 J respectively.