The electronic structure and the optical properties of Magnli phase titanium suboxide Ti8O15 are studied by using the plane-wave ultrasoft pesudopotential method based on the density functional theory. The band structure reveals that the energy band gap of Ti8O15 is reduced a lot compared with that of anatase TiO2, which is due to the fact that O 2p, Ti 3p and Ti 3d of Ti8O15 shift toward the left compared with those of TiO2, and a new electron energy level formed by the redundant electrons of Ti 3d and Ti 3p of Ti8O15 due to the lack of oxygen atom in lattice. The results from DOS analysis show that electron distribution near the Fermi level of Ti8O15 is different from that of anatase TiO2, contribution of O 2p to Fermi level decreases and that of Ti 3d increases. Compared with anatase TiO2 which only has high ultraviolet light absorption, Ti8O15 has high light absorptivity both in ultraviolet spectrum and visible spectrum, because its narrow forbidden band width results in the red shift toward visible-light region. The light absorptivity calculated results are consistent with those from UV-vis diffuse absorption test results of anatase TiO2 and Magnli phase titanium suboxides.