In order to improve the accuracy of measurement by arc image obtained by high speed camera, the modified Fowler-Milne method based on continuum spectrum was used. Based on the theory of spectrum, the relationships between emission coefficient and temperature were calculated. The emission coefficient includes spectrum line, continuum spectrum and total emission coefficient for 794.8 nm. The influence of continuum spectrum for FowlerMilne method was analyzed, and then the Fowler-Milne method was modified, arc image of spectra line in 794.8 nm was captured by high speed camera. The temperature distribution of direct current-tungsten intert gas arc welding was calculated according to the modified FowlerMilne method. Arc temperature field was compared before and after modification. The temperature field increases when continuum spectrum is taken into account. The result of spectrometer proves that the influence of continuum spectrum is small.