Abstract: As an important heat source in welding, TIG arc is widely used in manufacturing industry, especially for aluminum alloy joining processes.But up to now, the knowledge concerning the alternative TIG arc during the period of the current traversing zero as well as its influence on welding process are not clear enough.Based on a modern ICCD spectral instrument and time-control statistic sampling method, the dynamic variations of electron density during the period were observed in alternative TIG welding arcs for both the square wave and sine wave.The general rules and the features of the arcs were analyzed and discussed.It may enhance the knowledge on the arc process and its control level.