Abstract: The structural stability and electrical properties of (Tl_xGa_1-x)₂O₃ alloys were studied using the first-principles calculations based on density functional theory. Doping models of β-(Tl_xGa_1-x)₂O₃ and C-(Tl_xGa_1-x)₂O₃ with different concentrations were constructed by replacing the cations in β-Ga₂O₃ and C-Tl₂O₃. The structures of different models were optimized, and their thermodynamic stability, lattice constant, energy band structure, and density of states were analyzed. The calculation results show that Tl doping will lead to larger lattice parameters of β-Ga₂O₃, a larger enthalpy of formation, and lower stability. But in the β-(Tl_0.5Ga_0.5)₂O₃ structure, all Tl atoms occupy octahedral sites, and all Ga atoms occupy tetrahedral sites, forming a structure similar to an ordered alloy with the lowest local enthalpy of formation. When the Tl concentration is less than 66%, the formation enthalpy of the monoclinic phase structure is lower than that of the cubic phase structure, which is the preferred structure in terms of synthesis; the higher Tl concentration cubic phase structure is the preferred structure. As the Tl concentration increases, the bottom of the conduction band gradually shifts to a lower energy range, resulting in a gradual decrease in the band gap of the β-(Tl_xGa_1-x)₂O₃ system. From the analysis of the density of states, with the increase of Tl concentration, the bottom of the conduction band is gradually occupied by the Tl-6s state, resulting in a smaller band gap width. The above studies have shown that different concentrations of (Tl_xGa_1-x)₂O₃ systems have different preferred structures in terms of thermodynamic stability and controlling the appropriate doping concentration is an efficient way to achieve tunable electrical properties of (Tl_xGa_1-x)₂O₃ materials.