Abstract: A simplified calculation method of the earth atmospheric spectrum transmissivity and the IR radiance arrived at the top of atmosphere is introduced. Spectral range is from 4μm to ∞μm. The Elsasser band model and empirical parameters are used to calculate the absorption of H2O, and O3. The continuous absorption of H2O is calculated by LOWTRAN-6 empirical equation. The calculated results of transmissivity are coincided with those of LOWTRAN. Based on the simplified transmissivity model, a radiative transfer regression model is set up, and relevant software is developed. It is applied in data processing of satellite outgoing longwave radiation, and good results are obtained.