Abstract: In ramjet combustion chambers, carbon dioxide (CO₂) produced by the combustion of carbonaceous fuel enters the chamber together with boron agglomerates. In order to investigate the effect of CO₂ concentration present in an oxygen-containing atmosphere on the combustion characteristics and oxidation mechanisms of boron agglomerates, we used a laser ignition system, an X-ray diffractometer (XRD), and a thermogravimetric-differential scanning calorimetry combined thermal analysis system (TG-DSC) were used. Single-particle boron was tested in the laser-ignition experiments as the control group. The ignition experiment results showed that with a fixed O₂ concentration of 20%, when the particle temperature reaches the melting point of boron, increasing CO₂ content causes the combustion process of boron agglomerates to transition from single-particle molten droplet combustion to porous-particle combustion. Furthermore, XRD analysis results indicated that the condensed-phase combustion products (CCPs) of boron particles in a mixed atmosphere of O₂ and CO₂ contained B₄C, which is responsible for the porous structure of the particles. At temperatures below 1200 °C, the addition of CO₂ has no obvious promotion effect on boron exothermic reaction. However, in the laser-ignition experiment, when the oxygen concentration was fixed at 20% while the CO₂ concentration increased from 0 to 80%, the maximum temperature of boron agglomerates rose from 2434 K to 2573 K, the self-sustaining combustion time of single-particle boron decreased from 396 ms to 169 ms, and the self-sustaining combustion time of boron agglomerates decreased from 198 ms to 40 ms. This study conclusively showed that adding CO₂ to an oxygen-containing atmosphere facilitates boron reaction and consumption pathways, which is beneficial to promoting exothermic reaction of boron agglomerates at relatively high temperatures.