The defect geometry structure, band structure, density of states and optical properties are calculated in β-FeSi2 system and β-FeSi2 containing Si vacancies by density functional theory of potential plane wave method. The optimized results showed that Si vacancies result in the local lattice distortion, the narrower band gap and a new intermediate band between valence band and conduction band. The slight upward shift of Fermi surface due to Si vacancies leads β-FeSi2 to become a p-type semiconductor. The calculations of optical properties indicate that the electronic structure of adjacent atom is changed, the static dielectric constant ε1(0) increases, the highest peak of ε2 moves to a low-energy region and the tiny red shift of absorption coefficient takes place due to the Si vacancy .
