The energy band structure, density of states and optical properties of undoped Mg2Si and Nd-doped Mg2Si were investigated by the first-principles pseudopotential plane wave method based on density functional theory. The results show that after doping Mg2Si, the band gap of Mg2Si decreases from 0.290 eV to 0 eV, the conductivity improves. Undoped Mg2Si begins to have absorption ability when photon energy is greater than 0.9 eV, while Nd-doped Mg2Si begins to absorb photons with energy of 0.2 eV, the absorption of infrared electrons is improved after doping. The absorption coefficient and reflectivity of doped Mg2Si decrease, which indicates that the light transmittance of doped Mg2Si increases. The calculated results provide a theoretical basis for the application of Mg2Si materials in optoelectronic devices.