The present article describes the experiment of airborne Fourier transform infrared (FTIR) spectrometer detecting the background radiation of earth and atmosphere. Considering the surface radiation and atmospheric absorbance and emission, the expression of radiations measured by airborne FTIR is given. It is indicated that the background radiations depend on four parameters: surface temperature, surface emissivity, atmospheric transmission and temperature. Based on theoretic analysis the measured spectra are compared and analyzed according to the differences in altitude and surface. Spectral characters of infrared background radiation are described.