In order to study the influence of afterburning on the infrared radiation of a liquid rocket exhaust plume, a model was founded which could calculate afterburning flow field and infrared radiation of a liquid rocket exhaust plume. The FLUENT was used to calculate the afterburning flow field of a liquid rocket exhaust plume, the finite rate chemistry reaction model was adopted to calculate the afterburning reaction. Then, a narrow band model based on the HITEMP data base was employed to calculate the radiation parameters of plume gas. Lastly, the finite volume method was used to solve the radiation transfer equation. The feasibility of this model was testified by comparing the calculated spectral radiation intensity of the Titan IIIB exhaust plume and the result of (American) National Aeronautics and Space Administration. By means of this model, the influence of afterburning on the infrared radiation of a liquid rocket exhaust plume was calculated. The results show that, afterburning is able to increase the infrared spectral radiation greatly, and the average increase radio of the two domain radiation bands 2.5~3.0 μm and 4.2~4.7 μm achieve 30.8% and 28.3% respectively. Therefore, the afterburning should be taken into account when calculating the exact infrared radiation of a liquid rocket exhaust plume.