First, a numerical simulation of the fluid field is performed in the opposite direction of the airplane by using CFD software - FLUENT, so the distribute situation of the temperature is attained. Then, a model of the radiation from the nozzle of the turbojet engine and exhaust gases of the airplane is built. The total radiance and that in the above IR band are obtained by FLUENT, which provides a basis for the IR guidance air-defense weapon system design and application.