In this paper, Narrow Gap Metal Active Gas (NG-MAG) arc welding system was introduced, aiming at efficiently jointing AISI 316LN thick plate used in International Thermonuclear Experimental Reactor (ITER) device. Effect of heat input on cryogenic impact toughness of the WM was investigated and suitable welding parameters were explored to optimize the cryogenic toughness of weld metal (WM) and avoid weld defects simultaneously. Impressively, the maximum low temperature toughness of the NG-MAG arc welded WM could reach more than 100J, which could be comparable with that of the Tungsten Inert Gas (TIG) arc welded WM. More importantly, the NG-MAG arc welding showed much higher efficiency than the counterpart TIG. The less compositional segregation, and smaller dendrite size due to the lower level of heat input in NG-MAG arc welding were believed to play the critical role in enhancing the cryogenic toughness.