Abstract: Within a typical chemical oxygen-iodine laser(COIL), the residual heat could cause heat blocking in the pressure recovery system due to the chemical reaction in the laser cavity and the diffuser, which contributes to the failures of the diffuser start-up and the uniformity deteriorations of the supersonic flow in the cavity. For the pressure recovery performance, the total pressure losses due to the reaction could be up to 15%. By numerical simulations of the reaction flow in COIL cavities and supersonic diffusers, we investigated different configurations of the COIL supersonic diffuser, focusing on insert section length, number and shape of wedges, and diffuser length. The results show that under the adverse effect of residual reaction, the performance of the pressure recovery system can be improved by optimizing the insert section, adjusting the wedge length, and removing the semi-wedges in the diffuser side wall. The optimal diffuser geometry proposed in this paper proves to be heat-blocking-free, where the supersonic flow in cavity is not affected by the oblique shock waves due to separation. There is no separation in the choke of the diffuser. The separation area in the diffuser side wall decreases, and a more uniform outflow is achieved. For the pressure recovery system and COIL configuration in the paper, the optimal geometry achieves a total pressure recovery ratio of 0.426 and a static pressure ratio of 3.75, which are 25% better than the original design.