We calculated the electronic structure and optical properties of semiconductor material Ru2Si3 epitaxial on Si (001) that the epitaxial relation is Ru2Si3 (100)//Si(001) with Ru2Si3 [010]//Si[110] by using the pseudo-potentials plane-wave method based on first-principles methods.As shown by the calculated results,when the lattice parameter α is between 1.087 nm with 1.099 nm,the band gap of orthorhombic Ru2Si3 increases with the increase of α.The system is in the stable conduction as well as Ru2Si3 is a direct semiconductor with the band gap of 0.773 eV when the lattice parameter is 1.093 nm.The valence bands of Ru2Si3 are mainly composed of Si 3p,3s and Ru 4d,the conduction bands are mainly composed of Ru 4d and Si 3p.The calculated real part and imaginary part of dielectric function shows that the trend of change is similar on the whole between the epitaxial stable conditions and its neighbouring points.However,the curves of epitaxial Ru2Si3 drift toward low energy and the number of dielectric function peak decreases as well as the intensity bocomes strong obviously.