The martensite SA213-T91 and bainite SA213-T23 high-temperature resistant steels were applied to the heating surface of the ultra supercritical thermal power unit boiler.The weld metal microstructures and welded joint performance between the two kinds of dissimilar steels were analyzed.The main reasons of the welding defects such as hot crack,cold crack,brittleness and decrease in toughness were discussed during the welding of the dissimilar heat-resistant steels of SA213-T91 and SA213-T23 in boiler manufacturing and processing operation. The welding materials were chosen and welding procedure was made according to the base metals.