ABSTRACTIn building's cooling load calculation, solar heat gain through transparent envelope is calculated by using solar heat gain coefficient which is a thermal performance parameter of window. In traditional buildings, window-wall ratio is small so it's is assumed that the incoming solar radiation can't escape through the window again. But this hypothesis isn't suitable for glazing façade buildings. To calculate the escaped solar energy ratio, a solar radiation model is established on the basis of radiosity-irradiation method and calculated by using the commercial software of Matlab. The impact of time, room geometric dimensioning and absorptance of interior surfaces are evaluated. The numerical calculation results show that the escaped solar radiation ratio varies according to solar radiation incident angle in different times and its maximum value is 8.85% in summer solstice; compare to the width, the depth and height of the room affect the ratio significantly; the reflectance of the floor has greater impact on the escaped solar energy ratio than of other internal surfaces. Finally a fitting formula of escaped solar energy ratio is provided as a function of the ratio between the window area and the internal surface area and of the internal surfaces' absorptance.