Abstract: SiO₂–CaO–Al₂O₃ ternary inclusions are among the most common complex oxide inclusions in steel. Nevertheless, the chemical and physical properties of these composite inclusions, particularly with detailed composition changes, have not been sufficiently investigated. In this study, first-principles density functional theory calculations were used to determine the electronic, mechanical, and thermodynamic properties of two stable phases in the SiO₂–CaO–Al₂O₃ ternary inclusion system: anorthite (CaAl₂Si₂O₈) and gehlenite (Ca₂Al₂SiO₇). Based on the electronic density of states analysis and band structure calculations, oxygen atoms play important roles in the electron reactivity of both phases. Young’s modulus and Poisson’s ratios were calculated and compared with those of the SiO₂–CaO inclusions. The Young’s moduli of CaAl₂Si₂O₈ (101.32 GPa) and Ca₂Al₂SiO₇ (131.43 GPa) were close to the maximum and minimum Young’s moduli of the binary oxide inclusions, respectively. With increasing temperature, the Young’s moduli of CaAl₂Si₂O₈ and Ca₂Al₂SiO₇ showed slight increasing and decreasing trends, respectively, whereas the Poisson’s ratio decreased. Furthermore, the thermodynamic properties, particularly temperature-related thermal expansion coefficients, were also deeply investigated. The thermal expansion coefficients of both CaAl₂Si₂O₈ and Ca₂Al₂SiO₇ increased rapidly with increasing temperature in the low-temperature regime above 300 K. As the temperature increased, the increasing trend slowed. When the temperature reached 2000 K, the thermal expansion coefficients of CaAl₂Si₂O₈ and Ca₂Al₂SiO₇ respectively were 12 × 10⁻⁶ and 8.5 × 10⁻⁶ K⁻¹. These findings enhance the understanding of the physical nature of ternary inclusions in steels and provide a scientific foundation for analyzing their effects on steel performance using a more comprehensive inclusion database, thereby contributing to inclusion engineering in the development of materials with superior mechanical integrity.