In order to enhance the mixing of gas and air in the afterburning chamber of the solid rocket ramjet and improve the combustion efficiency, a swirl mixing device placed at the inlet exit of the solid rocket ramjet is designed. The accuracy of the swirl flow field calculation method was verified by the cold flow test of the solid rocket ramjet with swirler. A velocity uniformity coefficient based on the axial velocity distribution in the afterburning chamber was proposed. The influence of different blade angles and blade numbers of the swirl device on the flow field in the solid ramjet was numerically simulated, and the velocity uniformity coefficient and the total pressure recovery coefficient were used to evaluate the mixing performance of the afterburning chamber. The results show that the improved swirler designed in this paper can reduce the total pressure loss in the afterburning chamber through structural optimization. The blade angle of the swirling device affects the mixing effect of the afterburning chamber. When the blade angle varies from -30 ° to 30 °, the maximum difference in mixing effect is 7.20%. The six blades swirl device with a blade angle of 20° has the best mixing performance to improve the afterburning chamber. The mixing performance can be improved by up to 4.30%, and the total pressure loss can be reduced by 1.70%.