The molecular absorption region has a great advantage on space target detection from the altitude of satellite. The background radiation of the molecular absorption band in infrared region was calculated using the moderate spectral resolution radiative transport model MODTRAN5 at different spectral resolutions. All resolution results of three absorption bands were convolved with the same spectral response function to make an intercomparison and the results show that: for the 4.3 μm region the maximum relative deviation exceeded 10% and the 0.2 cm-1 spectral resolution results had an excellent agreement with the line-by-line calculation; the relative deviation between high and low resolution is rather small at 3.2 μm and 2.7 μm region; the maximum deviation at 2.7 μm region was below 5%. Further intercomparison was carried out between the deviation of MODTRAN4 and MODTRAN5 at 1 cm-1, 5 cm-1 spectral resolution respectively and the line-by-line calculation and the results show that the low resolution MODTRAN5 radiation calculation may result in considerable bias at 4.3 μm region.