The characteristics of GTA (gas tungsten arc) under high pressure has been studied in this paper to reveal the special arc phenomenon from the ambient pressure. A totally new hyperbaric GTAW experiment system has been built, including a hyperbaric environment simulator (0.1-3.5 MPa), an automatic GTAWmachine with cooling, the moving equipment, a camera with filter and a central control unit. The arc morphology, the electric static characteristic, the arc voltage and arc length are studied with the increment of ambient pressure from 0.1 to 3.5 MPa, respectively. The relationship between the ambient pressure and the arcvoltage is deduced according to the balance of heat-input and heat-output. The deduced function also verified the results from the welding experiment.