Abstract: A theoretic model is proposed based on the characteristics of radiation superposition for calculating the radiative intensity of clear atmosphere background. The model was used to calculate the spectral radiation luminance of the clear atmosphere background, and an iterative method was used to resolve the radiative transfer equation and perform the dynamic stratification of the atmosphere. The comparison between the results of the model computation and the well-known 6S software computation shows that this model and calculating method are accurate for the computation of the spectral irradiation luminance in the clear atmosphere background. And the results show that the spectral luminance of the clear atmosphere is high at short wavelength and weak at the strong absorption IR band.