Abstract: Based on radiative transfer equation, using the experiential formulae of air mass and atmospheric aerosol optical depth and atmospheric molecule parameters obtained by HITRAN database, an approximate algorithm for clear sky spectral luminance was given, which could be used generally. Clear sky spectral luminance was computed by the approximate algorithm model. Computation results indicate that there are distinct absorption lines of O2 and H2O on the curve of clear sky spectral luminance. The luminance of clear sky radiation is higher at shortwave band, while the spectrum for small sun angle shifts to longer waveband. The sky radiation luminance adjacent to horizon is higher.