This paper suggests the formula about sky luminance distribution on pseudoovercast sky. After testing and verifying,it is a fact that this formula and actual measure about pseudo-overcast sky luminance distribution are nearly identical. This paper also suggests unified fornula expressing four types of sky luminance distributions(CIE clear sky,pseudo-clear sky,pseudoovercast sky and CIE orercast sky). As a result,it is of great advantage to the calculation of vertical illuminance.