Abstract: By collecting spectrum of TIG welding arc, the spectral distribution in 200~1 100 nm was studied and the line spectra in the radiation were calibrated. Based on the character of arc plasma radiation, the radiation of TIG welding arc was analyzed. Furthermore, a method for diagnosing welding quality with line spectra of special elements was also provided. Through analyzing the spectral line (zone) of certain element, the spectral lines (zones) related to welding quality were selected. Experimental results show that the welding quality can be diagnosed with the spectral signal in the selected spectral zone, which varies during the welding process. Different disturbances have their characteristic diagnosis spectral zone.