Abstract: STEP series PWR fuel assembly is independently designed and developed by China,with full independent intellectual property rights. The STEP series fuel assembly consists of top nozzle, skeleton and fuel rods. Top nozzle welding is the key manufacturing process for PWR fuel assembly, which decides the mechanical properties and physical dimension of the top nozzle. Welding process test for the top nozzle of a fuel assembly was carried out. The welding joint of the top nozzle part was the form of no groove butt joint, the welding process with first manual spot welding then automatic welding with constant velocity was adopted. The appearance, metallography, intergranular corrosion and penetration of the top nozzle samples were tested, and the welding process parameters which meet the technical requirements were explored. The process difficulties such as melting depth and thermal deformation were analyzed, and the idea of process optimization and improvement was put forward.