External shading system of windows can reduce the indoor solar heating gain. Previous studies did not take the change of diffuse radiation into account. However, in fact, the change of diffuse radiation should not be neglected because of the existing of shading devices. Especially to those blinds, most direct radiation can be shaded while most diffuse radiation can reach the indoor space. Thus, a detail calculation model was established for diffuse radiation. With the analysis of the thermal characteristics of direct solar incidence on the blinds, a mathematic model was developed to calculate the transmittance and reflectance of the solar diffuse radiation. Based on this model, the relevant algorithm was developed.