Abstract: Tube to tubesheet welding technology of nickel-based alloy 690 was experimentally studied for the steam generator of AP1000 nuclear power stations. Using the process of all-position automatic inert-gas tungsten-arc welding, qualified weld joints were obtained in the single-layer single-pass self-fusion way with 95%Ar+5%H₂ as the shielding gas, and by separately setting welding parameters for different sections of each weld. Meanwhile, macroscopic and microscopic inspections were also carried out to the tube to tubesheet joints welded with different compositions of shielding gas and different heat inputs. Results show that for same welding parameters, different shielding gas would result in different sizes of effective throat. The risk of root crack would increase with the rise of welding heat inputs.