Abstract: An underwater welding robot system for large-scale structure is introduced for the environment of underwater welding and the system is composed of a robot mechanism, laser vision sensor, control system and welding system. The robot mechanism is composed of a pedrailed mobile platform and a torch adjustment mechanism and which can move flexibly for meeting the requirements of underwater welding. The components of sensor are selected and the optical path is also designed for the laser vision sensor to automatic welding seam identification. The PLC control system for the welding robot is designed and the coordinated control method is also adopted to achieve seam tracking accurately. And the underwater welding test platform is also completed. The results show that the robot operates steadily, reliably and the satisfactory welding quality is also obtained.