This paper presents the quantitative measurement to thesky luminance distribution with the help of the figure and numericalsystem of disposal,and suggests that the actual sky,with exception ofa few particular weather conditions,such as rain in which artificiallighting is needed,may be divided into CIE clear sky,pseudo-clearsky,pseudo-overcast sky and CIE overcast sky according to the skyluminance distribution,and verify the sky classification is reasonablewith fuzzy mathematics.