Electronic structures of pure and Al-doped SrTiO3 (STO) were investigated by first-principles calculations based on the density functional theory. For Sr1−xAlxTiO3 (x = 0.125, 0.25), Al impurity bands were observed at the Fermi level and the band gap of SrTiO3was effectively tuned. According to the calculated densities of states, there was obvious hybridization between O-2p and Ti-3d orbitals. The A-site ions in the ABO3 perovskite structure played a key role in the tuning of the band gaps. When Sr was replaced by Al, the covalent bond of Ti-O was strengthened. The results had implications for the experimentally observed photocatalytic functionalities of SrTiO3.