Abstract: The characteristics of GTAW (gas tungsten arc welding)arc under high-pressure air condition were studied for the first time using a homemade underwater welding simulation platform. It is revealed that the GTAW arc under high-pressure air environment has following distinguished features:its safe pressure range for underwater welding might be up to 0.7MPa;its voltage is from 12 to 19 V, a little higher than those of argon arc under argon surrounding;its static characteristic curve is assurgent when welding currents exceeds 50 A.The column electric field intensity of GTAW arc and the voltages of the cathode and anode are augmented as the air pressure becomes higher, which results in an upward shift on the static characteristic curve with an ascend rate about 5 -10 V/MPa.Base on the experimental data, a mathematical model of GTAW arc voltage under high-pressure air condition was founded.The model is useful to analyze and calculate the effect of the arc length, air environmental pressure and welding current on GTAW arc voltage.