Abstract: We addressed the narrow gap tungsten inert gas (NG-TIG) welding of the vacuum chamber of China fusion engineering test reactor (CFETR) assembled with 32 pieces of 316 L Austenitic stainless-steel.The mechanical properties of welded joints, especially in heat affected zone (HAZ), was mathematically modeled, theoretically analyzed and numerically simulated with ANSYS and experimentally evaluated.The influence of the welding conditions on the mechanical behavior was investigated with conventional mechanical probes for welding optimization.The sample joints welded under optimized conditions meet the stringent requirements of fusion reactor.For example, no obvious defects, including hot-cracks, incomplete fusion, voids and pores existed;the small heat input little affected the softening and grain growth in HAZ, though HAZ width has a major impact on overall strength of welded joint.We suggest that the results be of some technological interest in welding vacuum chamber of fusion reactor.