Temperature is an important parameter for assessing the thermal radiation damage of the ammunition. After the detonation of the ammunition will compress the surrounding air in a very short time and violently release a large amount of energy to the surrounding area. Along with the release of energy, ammunition media will be sharply warmed up, and the formation of the flame field, by measuring and analyzing the true temperature value of the flame field, it is possible to obtain the spatial thermal radiation damage effect of the explosion flame. Due to the explosion processs strong destructive and transient nature, the measurement of the explosion flame is mainly dependent on radiation pyrometer. In previous studies, scholars have developed corresponding radiometric devices for the measurement of the explosion flame. However, the developed devices can only measure the bright temperature field of the explosion flame at a single wavelength, a single wavelength bright temperature field cannot achieve the calculation of the true temperature value. This paper developed a multi-spectral thermal imager, the imager uses multi-amplitude spectroscopy technology that can realize the explosion flame true temperature field at the same time, different wavelengths of spectral imaging, and the use of a high-speed CCD camera for data acquisition, and finally based on multi-spectral radiometric temperature theory inverse performance of the explosion flame true temperature field. The multi-amplitude spectroscopy technology is accomplished by the long-range multi-aperture spectroscopy lens, divided into two main parts: the main imaging lens and the multi-aperture spectroscopy lens. The function of the main imaging lens is to image the long-range ammunition explosion field, the image which converges through a single convex lens to the rear of the multi-aperture spectroscopy lens. The multi-aperture spectroscopy lens has a built-in spectral light bar, the light bar can be set with different wavelength narrowband filters. When the image is through the narrowband filter on the light bar, the transmitted light will be the measured target of single-wavelength radiation energy, the use of multiple single-wavelength radiation energy can be through the multi-spectral radiation thermometry theory for the calculation of the true temperature value. In this paper, the long-range multi-aperture spectroscopic lens can image the explosion flame up to 500 m, and according to the actual demand of the lens light bar design for the four-split structure, while the light bar for the convenience of filter replacement into the pluggable form. The lens weighs about 0.75 kg and can be mounted directly on the high-speed CCD camera by the flange, which fully meets the requirements of field experiments. In order to verify the validity of the instrument, the explosion flame true temperature field test was conducted on 1.660 9 kg of TNT. The test results show that the maximum temperature value is 3 251 K at 0.1 ms after the explosion, and the true temperature field gradually expands with time passing, but the corresponding maximum temperature value gradually decreases; when the time is 0.6 ms, the maximum temperature is 2 483 K.