After establishing the infrared radiation ellipsoid modal of plume, the way for calculating IR (infrared radiation) of the fight plane was improved and the space distribution curve of IR in the 3～5 μm band of a certain type of fight plane was obtained. The curve showed that there were four directions which had the same maximum IR at the plane nose and the plane rear. Considering the IR absorption of O₃ in the 3～5 μm band, the IR transmitting obstruction by the ground and the detecting object in outer atmosphere. The IR atmosphere transmitted model in the 3～5 μm band was further improved with the concept of the IR transmission distance in the atmosphere.Taking the infrared system constructed by two kinds of infrared detectors with different normalized detection degrees as an example, the curve of the operating distance to fight plane varying with the detection direction was simulated. The calculation results show that the extreme value of the operating distance for the infrared system to the fight plane is not in the direction of the extreme value for the IR of the fight plane, and when the operating distance of the system is long, the influence of atmospheric attenuation on the operating distance will exceed the influence of the IR of the fight plane.