Abstract: Emissive spectrum diagnosis is important to study welding arc phenomenon. Limited by experimental condition in the past, the knowledge of spectrum dynamic diagnosis is not enough and most of the diagnosis results come from stable TIG arc. In this paper, based on modern ICCD spectrum instrument, a new type of arc spectrum dynamic diagnosis system is established and a new diagnosis method based on time-control and statistic-average is developed to observe AC dynamic welding arc continuously. The half-height width and wavelength shifting of AC TIG arc spectrum line can be observed, furthermore electron density and Ar atom excitation temperature can be obtained. This dynamic observation is quite significant to study the time-characteristic of arc (especially alternative current arc).