The calculation of exhaust plume's infrared radiation is an important part of the calculation of infrared radiation of an air vehicle. On the basis of an engineering calculation model of flow field in exhaust plume, the calculation of gas component parameters of a nozzle is implemented in terms of the chemical reaction of combustion. A fast calculation method of exhaust plume's infrared radiation is obtained by utilizing the C-G approximation method based on a Malkmus narrow-band model. The simulation result shows that the method can calculate the infrared radiation in exhaust plume fast and accurately. It can provide an effective approach for the calculation of infrared radiation of an aircraft in the simulation of an infrared guided missile.