The Mg₂Si polycrystalline films are fabricated on sapphire substrates by magnetron sputtering and post annealing treatment, and then the effect of annealing temperature (375~475 ^oC) on the crystal structure, surface morphology, Raman spectra and optical properties of the film is investigated. The X-ray diffraction (XRD) results show that when annealing temperature is 400 ^oC, the (220) diffraction peak of Mg₂Si film is the strongest, the crystal quality is the best, and no obvious MgO phase can be observed. The scanning electron microscopy (SEM) results show that all samples are regular hexagonal, and the annealing temperature has little effect on the morphology. The Raman spectra results show that all samples exhibit the characteristic peaks of Mg₂Si films (with F_2g vibration mode at near 256 cm^-1, and F_1u (LO) phonon mode at near 345 cm^-1), indicating that all fabricated samples are Mg₂Si films with good crystallization. The results of optical properties of the sample films show that, with the increase of annealing temperature, the optical band gap of the samples increases first and then decreases.