Abstract: Based on the structure of AP1000 steam generator, the effects of various shield gases on the welding shape and throat dimension of tube to tubesheet joint welded by all-position TIG welding were studied. The morphology and forming mechanism of the cracks appeared during welding under 100%Ar gas and higher heat input were also studied. The results show that under the same welding parameters, the weld pass was the narrowest at the atmosphere of 100%Ar and the widest under 100%He gas. The weld surface was the brightest under 95%Ar+5%H2 gas. The dimension of welding throat can meet with the technical requirements when welded under the atmosphere of 100%He, 95%Ar+5%H2 and 75%He+25%Ar. The optimum quality of tube to tubesheet joint was obtained when using TIG welding at the atmosphere of 95%Ar+5%H2. When improving heat input and lowering weld speed, the dimension of welding throat with 100%Ar could meet the technical requirements, but the probability of thermal cracks also increased. Those cracks were high temperature cracks.