Abstract: Hybrid shielding gas was often used in arc welding to improve its quality and speed. The research of its physical property is difficult, since the composition in hybrid shielding gas welding is complicated and variable. Arc plasma in argon-nitrogen tungsten inert gas (TIG) welding was chose as the research object, the double charge coupled device (CCD) experimental system was setup to obtain spectrum image of Ar I and N I, Fowler-Milne method was used to calculated the temperature and mole fraction distribution in 50%Ar + 50%N₂,80%Ar + 20%N₂ and 100%Ar arc plasma. The results showed that the radius shrinkage can reach 50%, and the temperature raise ccan reach 12% in 50%Ar + 50%N₂ arc plasma. It provides the theory basis for the research of weld seam forming essence through the analysis of the physical property of argon-nitrogen arc plasma.