Abstract: Through introducing oxygen and nitrogen elements from the outer gas of gas pool coupled activating TIG(GPCA-TIG) welding, deep penetration, high quantity and continuous welding are accessible. In order to obtain the dependence and mechanism of weld metal impact toughness on the introduced oxygen and nitrogen elements, GPCA-TIG welding for stainless steel SUS304 with the outer gases oxygen, nitrogen and nitrogen-oxygen gas mixture respectively were performed to gauge low temperature impact toughness of the weld metal. Then, a series of testing experiments including weld microstructure and chemical composition, nonmetallic inclusion and grain orientation were conducted. The results reveal that, the introduction of oxygen makes the low temperature impact toughness of weld metal decease obviously, nitrogen decease a little, while the low temperature impact toughness of weld metal can be increased a little conversely when oxygen and nitrogen are synchronously introduced. The main mechanism is proven that the synchronous introduction of oxygen and nitrogen elements leads to fine grain of weld microstructure, less increases of the contents of oxygen and nitrogen elements and the nonmetallic inclusions, much decrease of ferrite number in weld metal. Meanwhile, the high angle grain boundary number in austenite crystal and the matching property of grain orientations between ferrite and austenite are improved. All of these result in difficult cracking propagation and higher toughness.