Air diving are widely used in many shallow water areas, if applicable welding technology adaptive to such a condition developed, then operation cost of offshore structure repairs can be largely saved. In order to meet such a special requirement, an automatic GTAW welding process under high air pressures was researched. Firstly, the hyperbaric welding test chamber was designed and constructed in laboratory, and the welding machine was manufactured. Secondly, experiments under 1-7 bar air pressures were studied including linear welds and pipe butt welds, and high weld quality was obtained. Lastly, the hyperbaric welding sea-trial was carried out, and perfect 5G girth weld of sub-sea pipelines was produced.