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.