The Al-doped Mg2Si thin films on Si substrates are fabricated by magnetron sputtering method. Influences of doping levels on the compositions, surface topographies, roughness and optical band gap values of Mg2Si thin films are investigated with X-ray diffraction (XRD), scanning electron microscope(SEM), atomic force microscope(AFM) and spectrophotometer. XRD results show that the diffraction peaks of Mg2Si become stronger firstly, and then become weaker with increasing of Al doping amount. SEM and AFM results show that the crystallinity increases first and then decreases. Grain size decreases, and the roughness increases first and then decreases with increasing of Al doping level. The indirect transition band gap of the doped films ranges from 0.423 eV to 0.495 eV. The direct transition band gap ranges from 0.72 eV to 0.748 eV. The indirect and direct transition band gap are 0.53 eV and 0.833 eV before doping, respectively.