Based on the full-potential linearized augmented plane-wave method (FP-LAPW), we investigated the electronic structures and the optical properties, including dielectric function, absorption spectra, refraction and extinction of Sn1-xAlxO2 (x=0, 0.0625, 0.125, 0.1875, 0.25) via substituting for Sn with Al in SnO2 supercell. Calculated results show that the introduced Al could induce the band gap widening, and this can be attributed to the increased folded states, which lead to the contraction of the valence band and less Sn 5s states at the bottom of the conduction band. With the increasing of Al concentration, the band gap becomes wider, and the intensity of the first main peaks decreases. All of the optical spectra contain the imaginary part of dielectric function, the absorption spectrum, the refraction, and the extinction show blue shift corresponding to the increasing of the band gaps. Finally, it is expected that our results may inspire the future experimental research.