Based on the first-principles density functional theory, the electronic structures, densities of states and optical properties of the structures α-Nb5Si3, β-Nb5Si33 and γ-Nb5Si are calculated by using the local density approximation and plane wave pseudopotential method. The calculation results show that the valence band of Nb5Si3 near the Fermi energy is composed of Nb 4d, Si 3s, 3p orbits and the conduction band is comprised mainly of Nb 4d orbit; the optical properties of Nb5Si3 possess anisotropic characteristics, the static dielectric function ε1(0) of Nb5Si3 is about 207, and the refractive index n0 is 13. Their absorptions in a range above 15 eV approach to zero, showing the optical transparent behaviors.