The geometric structure, electronic structure and optical properties of intrinsic and K, Ti doped Mg2Si were studied by the first-principles calculation method. The calculation results indicate that the intrinsic Mg2Si is an indirect semiconductor material with a gap of 0.290 eV. After Mg2Si doped with K, the electronic transition mode changes from indirect transition to direct transition and the overall conduction type is p-type semiconductor. The band gap type of Mg2Si doped with Ti has not changed, which is still an indirect band gap, and it is n-type semiconductor. The static permittivity ε1(0) of the K and Ti doped Mg2Si increases from 20.52 to 53.55 and 69.25, respectively, which enhances the binding ability of the doped system to charges. After doping, both the absorption spectrum and the photoconductivity have a red shift phenomenon, which effectively expands the absorption range of visible light. Besides, doping significantly decrease the absorption coefficient, reflection coefficient and photoconductivity in the visible region, which leads to an increase transmission ability, and significantly improves the optical properties of Mg2Si.