By using the first principle pseudo-potential plane-wave method, the geometrical structure, electronic structure and optical properties of β-FeSi2 with doping rare earth (Y、Ce) are calculated and analyzed. The calculated results of the geometrical structure show that the lattice constants change, the volume of lattice reduces. Electronic structure calculation indicates that band structure near the gap becomes complex, and bandgap becomes narrow obviously. The total density of state changes, and the density of states for Y-4d and Ce-4f are mainly contributing to the Fermi energy level. Optical properties calculation indicates that the static dielectric constant increases, the peak of the imaginary part of dielectric function ε2 decreases and moves to a lower energy, the refractive index n0 increases significantly and the peak of k decreases. These results offer theoretical data for experimental study β-FeSi2 doped with rare earth modification.