By using the first principle pseudo-potential plane-wave method based on the density function theory, geometrical structure, electronic structure and optical properties of Co-doped β-FeSi2 are calculated and analyzed. The calculated results of the geometrical structure and electronic structure show that the lattice constant a increases while b and c have little change, the volume of lattice expands, the band structure changes from indirect to direct and the band gap reduces from 0.74 eV to 0.07 eV with increasing of Co from 0 to 0.25. Moreover, it was found that Co-doping can increase the volume of β-FeSi2, which also reduces the band gap. Optical properties calculation indicates that after doping Co, the imaginary part of the dielectric function ε2(ω) moves to a lower energy, the intensity in optical transition decreases, the optical absorption edge generates a red shift, and the optical band gap decreases with increasing concentrations of Co. These results offer theoretical data for design and application of optoelectronic material of β-FeSi2.