Abstract: In order to study the injection field of a chemical oxygen iodine laser(COIL),the injection flows in supersonic COIL were simulated by solving the Reynolds averaged Navier-Stokes equations and species continuity equations based on four-stage Runge-Kutta time-stepping scheme on structured viscous mesh.The simulations were carried out with AUSM+up scheme employing the k-ε turbulence model.The finite-rate reaction model was employed with thermodynamic perfect gas properties for chemical reaction.The stiffness problem of chemical source was overcome by a relaxation iterative scheme.Transverse He-I2 injection test case was simulated with this method.The results were in good agreement with the result of AeroSoft's GASP.The supersonic COIL flows with He-I2 injection were also simulated.The distribution of the mass fraction and averaged small signal gain were calculated.The results showed that the averaged small signal gain increased with the increasing of O2(1△) yields or with the decreasing of the water vapor.The averaged small signal gain of a COIL with two spouts is bigger than that of a COIL with one spout.