Abstract: The Ionization fraction is one of the important indexes of arc characteristics. The analysis of ionization fraction on arc of hyperbaric GMAW provides a new perspective for the research of welding process stability. The spectral of the points in hyperbaric GMAW arc were tested by a spectrometer with different environmental pressures. The arc ionization fraction of the tested point was calculated by Boltzmann method and Saha thermo ionization formula. According to the experimental data, the effect of environmental pressures on arc ionization fraction was analyzed. The results showed that with the increase of environmental pressure, the arc ionization fraction decreased remarkably, and the proportion of charged particles in hyperbaric GMAW arc space decreased. It is more difficult to maintain the stable arc, and the stability of hyperbaric GMAW process decreased. Additionally, it is helpful to increase welding current to improve the stability of hyperbaric GMAW process.