Abstract: 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.