Abstract: On the condition of 100 A welding current the frequency of optical emission from argon plasma in a hollow cathode has been recorded as 200~1 000 nm with water cooled copper anode.Spatial distribution (along the arc length) of hollow cathode vacuum arc spectrum is also given.The observed spectra shows that strong Ar Ⅰ and Ar Ⅱ lines are superimposed on a weak continuum.Large amounts of Ar Ⅱ spectral lines apppear in the region of 350~510 nm,which results in a blue hollow cathode vacuum arc.All the results indicate that the spectral line can be used to analyze the mass and energy transfer in hollow cathode vacuum arc welding plasma arc.