Abstract: During AC TIG welding of aluminum alloy, the physical phenomena of welding arc plasma at electrode negative(EN) and electrode positive(EP) periods and the characteristic of energy distribution at anode and cathode were investigated by observing the spectrum and founding the mathematical models.Furthermore, cathodic cleaning effect increases with the percentage of electrode positive.The dependence of electron density on time in the whole cycle is obtained with the theory of spectral line Stark broadening, and the characteristic of energy distribution is analyzed through the mathematical models.The mechanism of emitting electrons, cathodic cleaning and the quality of weld is better understood by observing the spectrum of argon, aluminum and oxygen respectively during EN and EP periods.