The goal of this paper is to analyze the differences of atmospheric radiative distributions between the scattered-cloud sky and the clear sky by simulations of a 3D radiative transfer model. The two parameters, radiance and radiative ratio of 450-nm to 650-nm radiance, are used to express the differences. Analysis results show that the radiative value of atmosphere will suffer the effect of surrounding cloud and the degree will depend on the parameters like cloud cover, cloud optical thickness as well as aerosol optical depth (AOD). It is found that radiance in most regions varies between ±2％ in the cloudy sky when the cloud cover is small, in comparison with the same fields in the clear sky. It indicates that the threshold value obtained from the 1D radiative transfer model, used to recognize cloud from sky in the all-sky image, is applicable for the inhomogeneous cloudy sky. Moreover, the “non-cloud” radiance in the cloudy sky can be used to make other retrieving researches like the AOD retrieval.