As an advanced ecological friendly semiconducting material, the researches on the preparation methods and optical properties of Mg2Si film play a fundamental role in the applications and development of Mg2Si films. Semiconducting Mg2Si films were prepared by electron beam evaporation deposition of Mg film onto Si (111) substrate and subsequent heat treatment under Ar gas atmosphere. X-ray diffraction (XRD), scanning electron microscopy (SEM) and spectrometer were used to characterize and analyze the obtained Mg2Si films. Effects of heat treatment time (3-7 h) at 500 ℃ under Ar gas pressure (200 Pa) on the formation of Mg2Si films were investigated. The XRD and SEM results show that semiconducting Mg2Si films are obtained by electron beam evaporation deposition and subsequent heat treatment at 500 ℃ for 3-7 h. 4 h is optimal heat treatment time to prepare Mg2Si films when heat treated at 500 ℃, and the compact Mg2Si film is obtained. The calculational results of infrared transmittance spectra of the Mg2Si films show that the indirect optical bandgap of the Mg2Si films is 0.9433 eV, and the direct optical bandgap is 1.158 eV. These experimental data are beneficial to the device research and development of the Mg2Si films in the preparation process and optical properties.