In laser-arc double-sided welding, the spectral characteristics of the arc plasma are calculated and analyzed by spectroscopic diagnosis. The results show that, compared with conventional tungsten inert gas (TIG) welding, the introduction of a laser changes the physical characteristics of the arc plasma regardless of whether laser plasma penetration takes place, and that the influence of the laser mainly affects the near-anode region of the arc. When the laser power is relatively low, the arc column tends to compress, and the arc spectral characteristics show no significant difference. When the arc root constricts, compared with pure TIG arc, the electron density increases by ～2.7 times and the electron temperature decreases by ～3000 K. When the arc column expands, the intensities of spectral lines of both the metal and Ar atoms are the strongest. But it is also observed that the electron density reduces, whereas there is no obvious decrease of electron temperature.