A detailed theoretical study on the band structure, electronic density and optical properties of β-FeSi2 under the isotropic stress is performed based on the first-principles pseudopotential method. The results show that the lattice constants of β-FeSi2 change with different stress. With the compression stress increasing, the densities of Fe-d and Si-p states decrease, and the conduction bands move to higher energy while the valence bands have little change that makes the energy gap widened; when the lattice is stretched, the densities of Fe-d and Si-p states increase, and the conduction bands move to lower energy while the valence bands have little change which makes the energy gap narrow down. The valence and conduction bands are traversed by the Fermi energy when stretch stress is -25 GPa that means the semiconductor β-FeSi2 converts to the conductor. Compressing lattice will lead to the blue shift, increase the absorption index and photoconductivity, and decrease the reflectivity. While stretching will result in the red shift, increase the static dielectric constant, refractive index n0 and reflectivity, and decrease the absorption index. Applying stress can effectively regulate the electronic structure and optical properties of β-FeSi2, which is an effective way to change and control the photoelectric transmission performance of β-FeSi2.