Abstract: S32750 super duplex stainless steel plates were welded by TIG under five different heat input. Microstructures of welded joints were carefully observed by means of optical microscope,laser confocal microscopy and scanning electron microscopy. N content of the weld and mechanical properties of welded joints were tested and analyzed in detail. The results showed that with the increase of heat input,the grain size of austenite in grain and on grain boundary increased gradually in the weld metal,while the ferrite content decreases linearly. The microhardness of welded joints under different heat input were higher than that of base metal. Nevertheless,the tensile strength under higher heat input was lower than that of base metal.