Abstract: Using bidirectional reflectance distribution function,the scattering of visible light from a spatial object the scattering of solar radiation which is reflected by cloud are calculated. Based on the theories of heat transmission and background radiation, the thermal balance equation is established according to the law of conservation of energy. Take balloon as an example, the temperature and radiation power which vary with time, height and geographic location are calculated.The general law of infrared radiation characteristic of spatial object is analyzed.