Abstract: Aimed at the demand of good quality underwater welding and maintenance for nuclear power station, a set of local dry automatic welding system has been developed,It is based on an underwater robot which is made up from a special high-power underwater welding source, a diving wire feeder, a mini drain cap, a welding robot and a special underwater welding torch. With DSC-level ARM microprocessor as its core and combined with the dual inverter topological, the welding source is of full-digital construction and multi-wave flexible output. Based on the armature voltage negative feedback and high-frequency chopping pulse width modulation, a compact diving wire feeding device has been designed and it has achieved the high-efficiency seal of the driving motor with the help of the dynamic and static sealing technology. Considering that the difficulty of local protection and deslagging in the welding area, a mini drain cap of duplex gas structure based on the principle of the convergent nozzle has been designed. Both the practical tests and the underwater welding experiments have showed that the underwater robotic local dry welding system is a perfect design. It can not only strike the arc stably and reliably in the shallow water environment but also form beautiful welding seam.