Abstract: To address the problem of insufficient strength-ductility synergy in ER308L deposited metal used for nuclear island equipment at a service temperature of 350 ℃, gas tungsten arc welding(GTAW) was employed to investigate the effects of welding speed, welding current and hydrogen addition in shielding gas on the microstructure and mechanical properties of the deposited metal. The results demonstrate that decreasing welding speed and properly increasing welding current prolongs the high-temperature dwelling time, promotes grain coarsening and improves the elongation after fracture. Hydrogen addition into the shielding gas further increases arc energy and weld penetration, which significantly improves the strength-ductility balance. Under the shielding gas of Ar-3%H₂, the tensile strength at 350 ℃ is higher than 380 MPa and the elongation after fracture reaches 27%. Nevertheless, when the hydrogen volume fraction rises to 5%, the precipitation of σ phase is promoted, leading to the deterioration of mechanical properties. The present study suggests that a proper amount of hydrogen is conducive to optimizing the weld microstructure and enhancing the high-temperature ductility, while excessive hydrogen induces the precipitation of brittle phases and deteriorates the mechanical properties