Abstract: During the refueling and overhaul period of nuclear power plant, it is imperative to inspect split pins of the control rod guiding tube in the upper grid plate. The split pin needs to be replaced underwater if any damage is found that makes it useless. After installation, a locking pin should be welded underwater to keep the split pin from rotating when the upper grid plate is under stress. This article focuses on the issues of underwater laser welding of split pins of the control rod guiding tube in the upper grid plate in nuclear power plant. A laser welding system was designed for underwater narrow spaces. The underwater laser welding process at a water depth of 2 m was investigated through a combination of numerical simulation and experimental research. The results showed that underwater laser welded joints of split pins had no defects such as pores and cracks. The welded joints could withstand a maximum tensile force of 10 445 N and a maximum torque of 120 N·m. The developed underwater laser welding system could achieve high-quality underwater laser welded joints of split pin, and the joint performance met practical service requirements.