The beam form has a direct effect on the quality and efficiency of flexible forming using a combined plasma arc. But it is difficult to directly measure relative parameters of combined plasma arc for its characteristics of high temperature and high energy density. In this paper, an image collection system was designed with a lens, a neutral filter, an interference filter and a CCD camera. And then, the acquired digital image of diagnostic face of plasma arc was implemented pre-processing such as removing noises, edge extraction, histogram equalization, contrast enhancement, enhancing morphological features. Furthermore, based on spectral diagnostics of plasma arc, the effects of working current, gas flow and the distance from the nozzle outlet to the anode on temperature distribution of combined plasma arc were also investigated. The results show that the temperature of combined plasma arc is much dependent on the working current, while is less sensitive to the argon flow rate and the distance from the nozzle outlet to the workpiece anode.