One-dimensional pre-mixed model for a COIL, in which the iodine molecular was substituted by iodine atomic, was established. Atomic iodine is generated via a series of chemical reactions of ClO2. NO and HI. The dependence of the vield of the intermediate atomic chlorine on total pressure was studied, as well as the influence of HI injection position on the yield of atomic iodine. The small signal gains both of the COIL with molecular and atomic iodine injection were compared. The system of atomic iodine injection is promising to increase the output power of COIL.