In the present article, the measurement principle of a multi-target multi-spectral thermometer developed previously is improved in that the brightness temperature approximation method is adopted to overcome the second measurement method initial value selection difficulty. The thermometer is used to measure the explosion true temperature of 3KG TNT explosive in open fields. The experimental results show that this pyrometer can measure changes in the explosion flame true temperature in the whole process, and functions well in the wavefront instantaneous temperature measurement and burning fireball temperature measurement. Finally, through the analysis of the factors which affect the thermometer measurement accuracy, it was concluded that the main error sources restricting the measurement accuracy improvement are still the true temperature algorithm error and the calibration method error. The analysis result has provided the direction for the high-precision thermometer development.