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.