This paper focused on the method to determine the physical thickness of a single-layer MgF2 film and its refractive index in the deep ultraviolet/vacuum ultraviolet spectral ranges. The single-layer MgF2 film was prepared on the B270 substrate by a Mo boat evaporation method. On the basis of reflectance spectra of the single-layer MgF2 film at various incidence angles, the refractive index and thickness of the film in 170-260 nm were determined by the simulated annealing algorithm and were compared with those of determined by ellipsometry. Experimental results indicate that the thicknesses of the MgF2 film are 248.5 nm and 249.5 nm by the simulated annealing algorithm and ellipsometry, respectively, which shows a deviation of 0.4%. Moreover, the refractive indexes of the MgF2 film at 240-260 nm obtained by two method mentioned above show a deviation less than 0.003. Obtained results prove that the method employing the reflectance spectral and simulated annealing is reliable for determining the refractive index and thickness of the MgF2 film.