Abstract: Welding arc and weld pool models were established by FLUENT software for spot and moving TIG welding of SUS304 stainless steel to investigate the effect of the surface-activating element oxygen on the weld shape and analyze the properties of argon arc and helium arc and their effects on the weld shape.The results show that the change of the Marangoni convection induced by different oxygen contents can be considered as one of the principal factors to increase penetration.The plasma drag force from the argon arc has obvious effect on the weld shape.Compared with the argon arc, the helium arc is more constricted, the welding current density is much greater and the much more heat flux is transferred into the weld pool, which increase the inward convection induced by the electromagnetic force, thus the deeper weld depth can be obtained.The calculated weld D/W ratio agrees with that of the experiment.