Abstract: Welding arc,as a heat source,converts electric energy to heat energy by gas discharging,and the scale of the arc ionization region as well as the effective range of the arc determines the distribution of its energy density.In this paper, the quantitative detection of argon atoms and argon-ion line intensity were achieved by emission spectroscopy diagnostic method, and the arc ionization region and the gas atom excitation radiation boundary scale were obtained.The measurement not only overcame the problem that the arc could not be quantitatively tested with camera due to exposure factors,but also could be used for observation of pulsed arc.The results provide the basis for better understanding arc physics and modeling the arc.