In order to study the changing law of the starch free radicals, the luminescence dynamics equation about the starch free radicals was set up based on the theory of biophotonics and the dynamics model of chemiluminescence. According to the relationship between luminous intensity and density of the free radicals, the annihilation function of the starch free radicals is obtained. By fitting the luminescence curve at different temperatures, the relative coefficient about the function is fund to be all above 0.98, which can describe the relaxation law about the free radicals accurately.