Based　on　high-time-resolution　spectral　instruments　with　an　image-intensified　charge-coupled　detector,　emission　spectra　of　AC　TIG　welding　arc　under　different　conditions　of　current　wave　forms,　such　as　square　wave,　trapezoidal　wave　and　sine　wave,　were　investigated　and　analyzed.　The　dependence　of　electron　density　on　time　was　obtained　using　the　theory　of　spectral　line　Stark　broadening.　The　experimental　results　show　that　there　is　a　time　delay　for　electron　density　alone　with　welding　current　crossing　the　zero　point.　It　is　discussed,　furthermore,　that　the　delay-time　has　relationship　with　velocity　of　current,　energy　input　and　the　rate　of　change　of　arc　input　power　with　respect　to　time　nearby　zero　point.　Thus,　non-Fourier　effect　is　revealed　and　the　condition　of　existence　is　discussed　during　thermal　energy　transport　of　the　arc-power　system.