The dissimilar weldment of P92/Inconel 625 is realized by the ultra-narrow-gap laser welding with filler wire and the microstructures and mechanical properties of the welded joints are systematically investigated. The results show that the quality of the welded joints have smooth welds, nice sidewall fusion, no pores and other defects. The welds possess a typical characteristic of solidification structure. The microstructure in the heat affected zone of P92 steels is martensite, which is divided into the coarse grain and fine grain zones. The micro-hardness in the fine grain zone of P92 steel at the welded joint interface is the largest, but it decreases after heat-treatment. The specimens fracture at Inconel 625 alloy in the tensile tests at the room temperature, but fracture at P92 steel in the tensile and stress-rupture tests at high temperature. The impact toughness of welded joints is between those of the two kinds of base materials.