In this page, arc imaging system was estabished with the double agglutination lens ,screen , dimmer glasses, aperture and precise mobile platform. The imaging system was aligned by laser, so that clear and magnified image could be shown on receiving screen basing on this system which is helpful for the specific position of measuring temperature . Spectroscopic diagnostics about welding arc plasma were carried out with this system and spectrometer HR4000CG-UV-NIR . Argon I spectral lines were chosen for calculation from welding arc spectrum collected and the modified double-line method was used to calculate arc temperature . The arc temperature measured with this method in this paper is similar to the result reported in the literatures. The experiment results show that the system established to measure the arc plasma axis temperature at fixed point is possible.