The combustion and flow performance in solid rocket ramjet's afterburning chambers were analyzed by CFD numerical simulation. The loss of afterburning chamber composes of adulteration loss and flow loss. The adulteration loss is on dominant when the afterburning chamber's aspect ratio is small (l/d <11), and the temperature uniformity of the outlet of afterburning chamber is low, so the performance of solid rocket ramjet is lower. With the length of the afterburning chamber increasing, the outlet flow condition of the afterburning chamber becomes more uniform, and the performance of the solid rocket ramjet gets tiptop. While, if the length extended even longer, the performance would be lower because of the flow loss augment. By the way, with the length of the afterburning chamber increasing, the flow performance of the nozzle of solid rocket ramjet would be in better condition as the flow condition is more uniform.