The urgent desire for Halon substitution propels the exploration of potential alternatives, because of the severe damage of Halons to the stratospheric ozone layer. In this paper, the thermal decomposition mechanism, as well as fire-extinguishing mechanism and performance of Trifluoroiodomethane (CF3I) were studied by density functional theory (DFT) calculation and experimental measurements, to analyze the practicability of this proposed Halon substitute. The thermal decomposition products of CF3I can react with active OH· and H· radicals to achieve the purpose of rapidly fire-extinguishing. Besides, through DFT calculation and reaction kinetics analysis, the fire-extinguishing radicals CF3· and I· are more easily generated during the interaction between CF3I and flame, which indicates the chemical- extinguishing mechanism and pronounced fire-extinguishing performance of CF3I. To explore its actual fire-extinguishing effect, the fire-extinguishing concentration (FEC) of CF3I was measured in cup burner. The FEC value of this proposed Halon substitute is 3.42vol% for extinguishment of methane-air flame, which is smaller than those of three HFCs and HFO-1336mzz(Z) and is comparable to that of Halon 1301. These findings suggest the promising applicability of CF3I in practical Halon replacement and the necessity of further evaluation.