The radiation spectrum of pyrotechnics’ burning flame was analyzed using transient spectrum radiometer. The working principle of multi-spectral thermometry was described. Combined with the radiation spectrum of pyrotechnics’ burning flame, the multi-spectral thermometer system was designed which had twelve working channels. The tester can choose the right working channels to calculate according to the radiation spectrum of the flame to be tested. The system is composed by optics part, electronic part, data acquisition part and data processing part. In this paper, the emissive power of black powder’s flame has been tested using the multi-spectral thermometer system. The burning flame temperature-time curve was showed after iteration calculation. Experiments indicate that the multi-spectral thermometer system can be well used to measure the flame temperature of pyrotechnics based on analyzing the emissive power when choosing the right working channels. This method lays a foundation for the research of combustion output characteristics of pyrotechnics.