Zenith luminance is an important parameter of sky background radiation. In order to acquire the zenith luminance characteristic, the zenith luminance was measured with sky background measurement system at somewhere in Xinjiang. Experimental data was analyzed. The result indicated that there was a certain relationship between the zenith luminance and sun high angle in sunshiny days. A relationship formula between the zenith luminance and sun elevation in sunshiny days was proposed. The validity of the formula was verified through the comparison of the calculation data with the experimental value. This relationship formula can be used to analyze the zenith luminance in real-time in sunshiny days. It is valuable to research sky background radiation luminance.