In order to research different diameters of air inlet pipe's influences to the inside flow field of a solid rocket ramjet engine on a certain length of the afterburning chamber, this paper simulated the mixing and burning conditions in the afterburning chamber of a designed solid rocket ramjet engine by using the standard k¯€ model and the species transport model. The results shows that under a certain length of the afterburning chamber and a certain in and out condition, there exists a corresponding diameter of the air inlet pipe, which can make the mixing and burning situation in the afterburning chamber best of all. Under the simulated condition in this paper the most suitable diameter is 30mm.