The optical constants, photoluminescence properties, and resistivity of $\mathrm{Al}\text{-}{\mathrm{Alq}}_3$ thin films prepared by the thermal co-evaporation method on a silicon substrate are studied with various Al fractions. A variable angle spectroscopic ellipsometry is employed to determine the optical constants in the wavelength from 300 to 1200 nm at incidence angles of 65°, 70°, and 75°, respectively. Both the refractive indices and extinction coefficient apparently increase with increasing Al fractions. The intensity of photoluminescence spectra gradually increases with decreasing Al fractions due to intrinsic energy level transition of ${\mathrm{Alq}}_3$ organic semiconductor in the ultraviolet wave band. The resistivity decreases from 42.1 to $3.36\mathrm{\Omega }·\mathrm{cm}$ with increasing Al fraction from 40% to 70%, resulting in a larger emission intensity in photoluminescence spectra for the 40% Al fraction sample.