As the optical constant of MgF2 over-coated Al mirror effects its optical performance greatly， this paper explores the method to obtain the optical constant. Three Al/MgF2 mirror samples covered by MgF2 coatings with different thicknesses were deposited on B270 substrates at room temperature by thermal evaporation method. The layer structures of the samples such as the thickness and the roughness were characterized by the grazing incidence X-ray reflectometry. The reflectivities of the samples at a incidence angle of 5° were measured in 105—130 nm region at the National Synchrotron Radiation Laboratory(NSRL). Finally， the optical constant of MgF2 for Al/MgF2 mirror was obtained by Fresnel formulas at given wavelength and corresponding reflectivity when the coating thicknesses of MgF2， Al， and the optical constant of Al have been known. The common intersection point of the curves of three MgF2 coatings with different thickness for Al/MgF2 mirror determines the optical constant of MgF2 coating at given wavelength. The analysis and comparison show that calculated reflectivity from optical constants of this work agrees well with measured reflectivity curves in 108—128 nm region.