he hyperbaric welding test chamber and the automatic GTAW machine were developed for high air ambient pressures. The arc static characteristics curves were tested under 0.1~0.7 MPa air ambient pressures, and the mathematical model of GTAW between arc voltage and parameters such as air ambient pressure, arc length, and current, was created. Based on arc static characteristics and the arc voltage mathematical model, main parameters of the automatic GTAW machine and the welding power were set, then typical position free forming joints of 16Mn steel plate under 0.1~0.7 MPa were manufactured, which were classified as class A in AWS D3.6M: 1999.