A mathematical model is proposed for calculating the solar gains through the multi-glazing facades (MGFs) with venetian blinds considering specular reflection. Net radiation method is used to establish the solar radiation energy balance equations in the overall MGFs layers. Then, a mixed method combines ray tracing and radiosity methods is used to determine the optical properties of the venetian blind in this system. Numerical results are compared with the optical properties of venetian blind calculated by the radiosity method and the experimental results. The comparisons show that the proposed model results fit better with the experimental results. Some important impact factors on the total solar transmittance of the MGFs are analyzed including the venetian blind geometric dimension W/S ratio, the slat reflectance, the slat shining factor, the slat angle, and the sun profile angle. Moreover, the effect of the proportion of the beam radiation on the total solar transmittance is also considered. The impact of the W/S ratio, the slat reflectance, and the slat angle are not limitless, when the solar profile angle is greater than (90 degrees-phi), the impact of them are not significant. Meanwhile, the impact of the shining factor and the proportion of the beam radiation depend on their intersection point. Before the slat angle intersection points, the higher the shining factor, the smaller the transmittance, but it is opposite for the proportion of the beam radiation. (C) 2016 Elsevier Ltd. All rights reserved.