The crystal structure, charge density distribution, band structure, density of states (DOS), and optical properties of Al-doped β-Ga2O3 (i.e. (AlxGa1-x)2O3) with different Al concentrations were calculated by the first-principles based on density functional theory. The calculated results for intrinsic and Al-doped β-Ga2O3 with various Al concentrations were compared. The results show that, as the Al concentration increases, both the lattice parameters and bond lengths of (AlxGa1-x)2O3 monotonously decrease, whereas the band gap progressively widens. It is found that an intermediate band exists above the conduction band minimum (CBM), which is mainly composed of Ga 4s and Al 3p orbitals. Al doping enlarges the band gap by introducing impurity energy levels in this intermediate conduction band. Meanwhile, the introduction of Al atoms shifts the density of states to high-energy side by nearly 3 eV, which also leads to an increase of the band gap. According to the optical property calculation results, a significant blue shift for both the imaginary part of the dielectric function and the absorption coefficient are observed after Al doping. This blue shift behavior is generated by the transition from the O 2p states in the valence band maximum (VBM) to the Ga 4s states in the CBM. Moreover, the blue shift is intensified with the increase of Al doping concentration. This investigation can provide ideas and theoretical guidance for the construction of optoelectronic devices based on (AlxGa1-x)2O3.