Train speed is limited by many factors, in which the off-line between catenary and pantograph has a great impact on power quality. Because of sliding contact mode, the pantograph and catenary are seriously worn. The arc generated by offline in pantograph and catenary is one of the main reasons of the abrasion on pantograph and catenary, but the arc temperature is very high, and it has ionization characteristics, so the general sensors cannot be placed in the arc to measure characters. Considering the arc’s feature, spectroscopy is important to study temperature and electron density of arc, and it is obviously advantaged. With this basis, researchers can find new wear-resistant material or arc-extinguishing method. We select specific spectrums of Cu as the studying object for catenary is made by copper alloy. We calculate arc temperature using Boltzman mapping method through spectral information experimentally measured. The result shows that temperature is not particularly high cause of short sustained time, and we know that the temperature is mainly affected by sustained time. Then we using deconvolution method to calculate Stark broadening, and according Stark broadening calculate electron density of the arc. The results show that the electron density of pantograph-catenary arc is not special different from electron density of welding arc. Spectroscopy is a convenient and accurate method in the calculating of temperature and electron density of arc.