The radiant temperature is one of the most parameters to show characteristic of radiant source. The emissivity of thermal radiator must be measured accurately to realize testing the radiant temperature. According to the character of flame radiation temperature during ammunition explosion and Plank's law,multi-spectral thermometry is used in this paper to fit the emissivity of flame spectrum, absolute radiometric calibration is done. An equipment based on multi-spectral thermometry is developed to detect flame temperature during ammunition explosion. The on-site experiment and data analysis are done, the accurate flame temperature is achieved by our equioment.