Abstract: CaO–SiO₂ compounds compromise one of the most common series of oxide particles in liquid steels, which could significantly affect the service performance of the steels as crack initiation sites. However, the structural, electronic, and mechanical properties of the compounds in CaO–SiO₂ system are still not fully clarified due to the difficulties in the experiments. In this study, a thorough investigation of these properties of CaO–SiO₂ compound particles in steels was conducted based on first-principles density functional theory. Corresponding phases were determined by thermodynamic calculation, including gamma dicalcium silicate (γ-C2S), alpha-prime (L) dicalcium silicate (\text α\,_\rmL ' -C2S), alpha-prime (H) dicalcium silicate (\text α\,_\rmH ' -C2S), alpha dicalcium silicate (α-C2S), rankinite (C3S2), hatrurite (C3S), wollastonite (CS), and pseudo-wollastonite (Ps-CS). The results showed that the calculated crystal structures of the eight phases agree well with the experimental results. All the eight phases are stable according to the calculated formation energies, and γ-C2S is the most stable. O atom contributes the most to the reactivity of these phases. The Young’s modulus of the eight phases is in the range of 100.63–132.04 GPa. Poisson’s ratio is in the range of 0.249–0.281. This study provided further understanding concerning the CaO–SiO₂ compound particles in steels and fulfilled the corresponding property database, paving the way for inclusion engineering and design in terms of fracture-resistant steels.