Abstract: The reduced activation ferritic/martensitic CLF-1 steel used in the nuclear fusion reactor test blanket module (TBM) is welded by 15 kW fiber laser, and the penetration welding of 17.5 mm thick CLF-1 steel is realized. A sound joint forming can be obtained by optimizing the process parameters, and there are no obvious defects such as cracks and pores within the welded joint. Furthermore, the microstructure and mechanical properties of the welded joints are analyzed. The experiment results show that the microstructure of the weld mainly contains the thick lath martensite. The microstructure of the heat affected zone mainly contains the tempered sorbate and martensite dual phase structure caused by secondary tempering of welding thermal. At the average room temperature and 550 ℃, tensile strength of welded joint is very high. The fracture position is located at the base metal. The microhardness of the weld is significantly higher than that of base metal, and there is no obvious soft zone at the heat affected zone. The weld joints exhibit favorable impact toughness, so it has very good comprehensive mechanical properties.