In the present paper, Boltzmann plot method was applied to analyze the temperature distributions of the arc plasma when the gas pool coupled activating TIG welding was at different coupling degrees with the outer gas being O2. Based on this study of temperature distributions, the changing regularities of arc voltage and arc appearance were studied. The result shows that compared with traditional TIG welding, the introduction of O2 makes the welding arc constricted slightly, the temperature of the arc center build up, and the arc voltage increase. When argon being the inner gas, oxygen serving as the outer gas instead of argon makes the arc constricted more obviously. When the coupling degree increases from 0 to 2, the temperature of the arc center and the arc voltage both increase slightly. In the gas pool coupled activating TIG welding the arc is constricted not obviously, and the reason why the weld penetration is improved dramatically in the welding of stainless steel is not arc constriction.