Infrared character of aircraft is one important factor to estimate the ability of breaching the defense of enemy,this paper firstly analyzed the method of infrared calibration with collimator and energy factors composing the sky background,propose one method of deleting energy of background within current image; furthermore,the paper points out that the parameters of calibration with collimator cant be directly applied to computing Infrared character of flying target,and based on that propose the method of deleting energy of background with another background image measured before mission.The projected area of high speed aircraft on infrared image is always small,thus the image can be segmented into two parts: target area denoted by A,background area denoted by B.According to the analysis of section 2,the energy of region A consists of not only thermal radiation of aircraft but also the aerosphere thermal radiation,environmental energy,these stray energies are also reflected in the gray level of background region.Then to obtain high precise infrared characteristics of aircraft,these stray energies should the eliminated,in this section we proposed one method dealing with stray energies with gray level of background.