The microstructure and properties of the welded joint were studied based on the tungsten argon arc welding of 2 mm thick alloy 800H. The characteristics of microstructure, mico-hardness, room-temperature and high-temperature tensile strength, fractography and intergranular corrosion of samples were analyzed. Results show that the crystallizing morphology on the weld seam is of columnar crystals and equiaxed grains, the grains of heat-affected zone (HAZ) grows bigger, and a small amount of TiN and (Fe,Cr)23C6 phases precipitate in the welded joint. The average hardness of base metal, HAZ and welding seam is 1730, 1526 and 1590 MPa, respectively. Moreover, room temperature tensile strength and elongation for alloy 800H is 565.0 MPa and 31.8%, respectively, which are both higher than the ASME standard requirement (tensile strength 450.0 MPa and elongation 30.0%), and ductile fracture happens at room temperature. The high temperature (650 °C) tensile strength and elongation of alloy 800H welding joints is 394.5 MPa and 15.5%, respectively, and the fracture is mixed mode at 650 °C. In comparison with welded joint of alloy 800H, the corrosion of base metal are much severer, with intercrystalline fracture of base metal and a few corrosion pits on the surface. The TiN precipitates in the base metal could result in pitting.