Through collecting the spectral signals of TIG and MIG welding arc with spectrometer,the arc light radiations were analyzed based on the basic theory of plasma physics.The radiation of welding arc distributes over a broad range of frequency,from infrared to ultraviolet.The arc spectrum is composed of line spectra and continuous spectra.Due to the variation of metal density in the welding arc,there is great difference between the welding arc spectra of TIG and MIG in both their intensity and distribution.The MIG welding arc provides more line spectra of metal and the intensity of radiation is greater than TIG.The arc spectrum of TIG welding is stable during the welding process,disturbance factors that cause the spectral variations can be reflected by the spectral line related to the corresponding element entering the welding arc.The arc spectrum of MIG welding will fluctuate severely due to droplet transfer,which produces “noise” in the line spectrum aggregation zone.So for MIG welding,the spectral zone lacking spectral line is suitable for welding quality diagnosis.According to the characteristic of TIG and MIG,special spectral zones were selected for welding quality diagnosis.For TIG welding,the selected zone is in ultraviolet zone (230-300 nm).For MIG welding,the selected zone is in visible zone (570-590 nm).With the basic theory provided for welding quality diagnosis,the integral intensity of spectral signal in the selected zone of welding process with disturbing factor was studied to prove the theory.The results show that the welding quality and disturbance factors can be diagnosed with good signal to noise ratio in the selected spectral zone compared with signal in other spectral zone.The spectral signal can be used for real-time diagnosis of the welding quality.