The emissivity of condensed-phase product particles was corrected by using impact factor of particle size. Combined with combustion model of aluminum, the emissivity of condensed-phase product particles and flash radiation model of pyrotechnic deflagration, the formula relationship between Al particles diameter and radiation intensities of pyrotechnic composite was put forward. It is concluded that the higher radiation intensity is, the smaller aluminum size of flash pyrotechnic composite is. Binary mixtures containing KClO₄ and Al with mass ratio of 50：50, and trinary mixtures containing KClO₄, Al and 3% epoxy with mass ratio of 50：50：3 were prepared, and their radiation intensities were determined experimentally. Results show that there is agreement between the theoretical conclusion and experimental results.