In order to reasonably evaluate the primary gas/air mixing combustion process in the secondary combustion chamber of solid rocket ramjet, a particle mixing degree and particle combustion efficiency based on particle mass concentration were proposed. The numerical simulation method of engine is established and verified by ground test. Aiming at the mixing combustion process in the afterburner under different working conditions, the changing trend of mixing degree and combustion efficiency in different definitions was compared by numerical simulation. The results showed that the proposed particle mixing degree and particle combustion efficiency showed the greatest correlation, and the average maximum information coefficient reached 0.9163 under different working conditions. Moreover, the primary gas/air mixing and combustion process show the following correlation laws: the particle combustion efficiency increases with the increase of particle mixing degree, and when the particle mixing degree exceeds a certain threshold, the particle combustion efficiency remains relatively stable.And the law is verified by experiments.