Abstract: This paper describes a newly developed local dry CO₂ gas shielded semi-automatic underwater welding method and its application.As discussed and emphasized by the authors,the protruding problems encountered with in underwater wet welding are poor visibility,high hydrogen content and fast cooling.The key to the improvement of welding quality lies in the minimization of these unfavourable factors.The features of the equipment,including its power source,underwater wire feeding box and welding gun are described.Carbon dioxide,the shielding gas,is used also to keep the water from entering the gun chamber.The method has successfully solved the problems mentioned above.The quality of the weld conforms to the code requirements of API 1104 and ASME IX.The said equipment can be operated by divers wearing different kinds of diving suits in the multi-position underwater welding of various types of joint.The method has been applied to the underwater construction of six pile-structures of an off-shore platform in the Bohai Sea,13.5M below surface,with satisfactory results.