The recent discovery of pressure-induced superconductivity in La$_{3}$Ni$_{2}$O$_{7}$ has established a novel platform for studying unconventional superconductors. However, achieving superconductivity in this system currently requires relatively high pressures. In this study, we propose a chemical pressure strategy via Sr substitution to stabilize high-$T_{\rm c}$ superconductivity in La$_{3}$Ni$_{2}$O$_{7}$ under ambient conditions. Using density functional theory (DFT) calculations, we systematically investigate the structural and electronic properties of Sr-doped La$_{3-x}$Sr$_{x}$Ni$_{2}$O$_{7}$ ($x= 0.25$, 0.5, 1) at ambient pressure and identify two dynamically stable phases: La$_{2.5}$Sr$_{0.5}$Ni$_{2}$O$_{7}$ and La$_{2}$SrNi$_{2}$O$_{7}$. Our calculations reveal that both phases exhibit metallization of the $\sigma $-bonding bands dominated by Ni-d$_{z^2}$ orbitals - a key feature associated with high-$T_{\rm c} $ superconductivity, as reported in the high-pressure phase of La$_{3}$Ni$_{2}$O$_{7}$. Further analysis using tight-binding models shows that the key hopping parameters in La$_{2.5}$Sr$_{0.5}$Ni$_{2}$O$_{7}$ and La$_{2}$SrNi$_{2}$O$_{7}$ closely resemble those of La$_{3}$Ni$_{2}$O$_{7}$ under high pressure, indicating that strong super-exchange interactions between interlayer Ni-$d_{z^2}$ orbitals are preserved. These findings suggest that the doped phases may provide a promising platform for exploring superconductivity, which requires further experimental validation.

Determining the electronic structure of La$_3$Ni$_2$O$_7$ is an essential step towards uncovering its superconducting mechanism. It is widely believed that the bilayer apical oxygens play an important role in the bilayer La$_3$Ni$_2$O$_7$ electronic structure. Applying the hybrid exchange-correlation functionals, we obtain a more accurate electronic structure of La$_3$Ni$_2$O$_7$ at its high-pressure phase, where the bonding $\mathrm{d}_{z^2}$ band is below the Fermi level owing to the apical oxygen. The symmetry properties of this electronic structure and its corresponding tight-binding model are further analyzed. We find that the antisymmetric part is highly entangled, leading to a minimal nearly degenerate two-orbital model. Then, the apical oxygen vacancies effect is studied using the dynamical cluster approximation. This disorder effect strongly destroys the antisymmetric $\beta$ Fermi surface, leading to the possible disappearance of superconductivity.