The electronic structure and optical properties of Mn4Si7 and Mn4Si7 doped by Mo were calculated by the density functional theory (DFT) based first-principles method. The calculation results show that the bandgap width of Mn4Si7 Eg=0.804 eV, and the bandgap width of Mo doped Mn4Si7 Eg=0.636 eV. Doping changes the electronic structure near the Fermi surface of Mn4Si7. The bottom of conduction band by Γ transfer to Y points to low-energy, under the direction of migration to the top of valence band offset to high-energy direction, narrow band gap. The calculation also shows that Mo doping Mn4Si7 increases the dielectric function, refractive index, absorption coefficient, photoconductivity and other optical properties.