The atmospheric transmittance is an important parameter affecting the solar radiation and surface thermal radiation. Based on MODTRAN radiation transfer model,the atmospheric transmittance in middle infrared (MIR) spectrum and the influences of atmospheric molecules and aerosol particles were analyzed in detail. Combined with six channels of MODIS set in MIR,the influences of absorption gases on channel transmittance were discussed respectively. The results show that water vapor and uniformly mixed gases are the two major factors causing the atmospheric transmittance attenuation. MODIS channel transmittance is strongly related to spectral response function band width and absorption band of atmospheric molecules.