Abstract: In this paper,by analysis of the small signal gain character of CW COIL,we predicted that there is a optimum ratio between O₂（¹Δ_g） and molecular iodine. Experi-mental result shows that the optimum ratio is about 50 to 60.Sequently,considering the key-chemical processes in COIL, and well-mixed working substance,a simplified one dimensional model for COIL is made out, the small sighal gain and one -dimensional distribution along the flow direction depending on the O₂（¹Δ_g）/Ⅰ₂ ratio also are studied. In the last, we obtained that when g_mL_e reached the top value depending on O₂（¹Δ_g）/Ⅰ₂ ratio,and the ratio is about 40 to 60, COIL works in the optimum condi-tion.