Abstract: Using the average parameters at the outlet of a mixing nozzle calculated based on experimental data of a chemical oxygen-iodine laser(COIL) apparatus without primary buffer gas as input conditions, the flow fields from the inlet of optical cavity to the outlet of diffuser for six different diffuser configurations were numerically simulated. Contours of different flow field parameters such as Mach number, static pressure, and total pressure etc. were obtained. The flow field characteristics and the total pressure recovery performance for different diffuser configurations were analyzed. The effect of back pressure at the diffuser outlet on flow field parameters was also studied. The results indicate that the diffuser with constant cross section has better pressure recovery performance for CO