The doping effects of Al2O3 on SiO2 (alpha-cristobalite) have been studied by first principles calculations, with emphasis on the structural, electronic and optical properties. Compared to pure SiO2 crystal, the electronic density of states (DOS) of the Al2O3 doped SiO2 show significant changes. The electron energy states corresponding to the newly emerged sharp DOS peaks are found to exhibit localized characteristics. which are mainly attributed to the unsaturatedly bonded 2p orbitals of O atoms in the -[Al-O-Al]- linkages. The optical properties of the Al2O3 doped SiO2 are studied by calculating the frequency-dependent dielectric functions. The electric dipole moment induced by the electron states near the top of the valence band is found to have significant effect on the optical absorption spectrum.