Abstract: In order to decrease dilution ratio and burning ratio of hard metal alloys and increase deposition rate in surfacing, the physical characters of carbon electrode arc constrained by argon have been investigated by using a diagnosis device of multi-line spectrum of plasma arc in this paper. The spectrum signal measured was transformed into electric signal and was magnified,and then it was feed into a A/D device. After it was transformed into digital signal,it was feed into a computer to be proceased. Therefore,the temperature, composition, pressure and their distribution were gained. The results show that the temperature of carbon electrode arc constrained by argon is higher than that of free carbon electrode arc,the arc gas is of deoxygenization,and the distribution of temperature and pressure of the arc is hom,ogeneous. These characters are benefit for the quality of bead welding.