With the help of CASTEP first-principles calculations, the different doping structures and thermal properties of the semiconductor AlxGa1-xN were carefully studied for better understanding the influence of the microstructure on the thermodynamic properties in group Ⅲ nitrides, and then provide data support during the design processes of ultra-high power devices. After the optimization of AlxGa1-xN structures, with the increase of Al composition (atomic fraction), the lattice constant, average bond length and lattice heat capacity linearly decreases. The calculation of properties results indicate that the introduction of Al component in GaN will introduce an impurity mode in the frequency band gap. As the concentration of Al component increases, the impurity mode widens and enters the low frequency range. The top frequency of the low frequency band increases with the increase of Al composition, and the top frequency of the low frequency band above 12.5% is larger than (1/2)A1(LO). From 300 K to 700 K, at the fixed temperature, with the increase of Al composition, the heat capacity of AlxGa1-xN alloy linearly decreases. The result in this paper provides a useful reference for the design of Ⅲ-nitride semiconductor high-power devices, such as AlxGa1-xN.