Abstract: The two-phase microstructure evolution of automatic TIG welded joints of ER2205 stainless steel under fixed welding parameters was studied. The results show that the TIG weld line subjected to a single weld heat effect has a ferritic phase ratio of about 50%~60%, whereas the ferrite content in the heat affected zone is as high as 70%; that with the progressing of the welding process, the ferrite contents in the weld and heat affected zone, affected by the heat from the subsequent weld passes, continue to decrease; that the austenite phase precipitates continuously in the weld bead during the subsequent welding process, the proportion of ferrite decreases continuously; that the ferrite formation of HAZ would be significantly affected by the weld pass of itself and the subsequent weld passes; that the ratios of ferrite in the middle passes of the weld are lower than those in the bottom and the capping passes.