It is difficult to obtain both sky luminance and solar radiation data, which greatly restrict the development and utilization of solar resources. In this paper, the functional relationship between sky luminance and solar radiation is established. Firstly, linear and polynomial functions were established for 145 regions of the whole sky. The results show that there is a nearly linear correlation between sky luminance and solar radiation. In 145 regions, 68.3% of the regions have close accuracy between linear and polynomial functions. Secondly, the linear and polynomial equations functions are established for the four typical orientations. The accuracy of the polynomial function in the eastern and northern regions is higher than that of the linear function, but in the southern and western regions they are equivalent. Finally, a unified equation is established for the whole sky, and the result shows the accuracy of the polynomial function is more accurate. This research can realize the mutual conversion between sky luminance and solar radiation. It can not only make up for the missing sky luminance data and solar radiation data, but also provide a theoretical basis for solar thermal, photovoltaic, building energy conservation and agricultural production.