By the multi-pass fiber laser welding technique with filler wire based on the ultra-narrow-gap grooves, the 50 mm thick SA508Gr.3Cl.2 nuclear power steel is welded, and the microstructures and properties of joints are tested and analyzed. The results show that there are no macro defects inside the welded joints. After heat treatment, the central organization of the welds in the middle of joints mainly consists of upper bainite and acicular lower bainite, the coarse grain area in the heat affected zone near the base material consists of high-temperature tempered martensite, and the microhardness of this area is 280 HV. The tensile fracture position of welding joints is located in the base material, which indicates that the tensile strength of joints is good. The impact toughness of the welds in the top, middle and bottom area of joints are smaller than that of the base material, but the fracture morphology of the impact specimen extracted from the welds has certain characteristics of toughness fracture.