Abstract: To improve the thrust of a solid fuel ramjet (SFRJ), a segmented-grain configuration was proposed by introducing an additional solid propellant grain into the afterburning chamber to increase the burning surface area and fuel supply. Based on the k-ε turbulence model and the Eddy Dissipation Model (EDM), a numerical model was established to investigate the effects of grain length in the afterburning chamber on the internal flow field characteristics and combustion performance. The performance of the segmented-grain SFRJ was also compared with that of a typical SFRJ. The results show that, with the increase of the grain length, the thrust and overall average regression rate increase continuously, whereas the characteristic velocity, combustion efficiency, and total specific impulse first increase and then decrease. With a further increase in grain length, combustion becomes increasingly limited by oxidizer supply, preventing the additional fuel from being fully oxidized. Under the present computational conditions and within the investigated range, 50 mm can be regarded as the critical grain length, at which the overall performance is optimal. Compared with the typical configuration, the overall average regression rate and thrust increase by approximately 26% and 14%, respectively, while the combustion efficiency increases from 79.05% to 83.73%. Therefore, the grain length in the afterburning chamber is a key parameter affecting the overall performance, and a comprehensive trade-off among the regression rate, thrust, and combustion efficiency should be considered in the design process.