Using atmospheric transmission model softwave Lowtran 7, the solar, atmospheric, sky and earth background radiation from 4 to 8 μm are calculated. According to the relationship between optical scattering theories on rough surface and bidirectional reflectance distribution function, the scattering of visible light from a spatial object is calculated. Based on the theories of heat transmission and background radiation, the surface thermal balance equation of spatical object is established according to the law of conservation of energy. Take balloon as an example, dependence of the temperature and radiation power on time, height and geographic location is calculated. The general law of infrared radiation characteristic of spatial object is obtained.