The effect of pressure and temperature on carbon particle combustion was studied by an improved Moving Flame Front (MFF) model. On the basis of the above discussion, the carbon particle combustion in solid rocket ramjet was analyzed. As a result, the carbon particle combustion mechanism in solid rocket ramjet was achieved. Moreover, numerical results were validated by direct-connect test of solid rocket ramjet. The result shows that most of carbon particle in the secondary combustion chamber burned near the inlet exit.