Passive radiative cooling (PRC) is a promising way to alleviate the global energy crisis by reflecting sunlight and dissipating heat through the atmospheric transparent window (ATW). Despite possessing a wide bandgap and complex phonon modes, the PRC performance of Mg₂Al₄Si₅O₁₈ is limited by phonon-polariton resonance. Herein, phonon engineering is integrated with bandgap engineering to design and synthesize a series of Mg₂Al₄Si₅O₁₈:xY³⁺ (x = 0%, 2.5%, 5%, 7.5%, and 10%) ceramics with excellent PRC performance. Density functional theory (DFT) identifies that Y³⁺ doping effectively suppresses phonon-polariton resonance and widens the bandgap, synergistically enhancing the PRC performance. The as-prepared samples exhibit high ATW emissivity (94.39%–98.39%) and high reflectivity (89.52%–94.77%) in the 0.4–2.5 μm range. Furthermore, the “cooling glass” coating successfully achieves a maximum temperature reduction of 16.5 °C and an average net radiative cooling power of 113.1 W·m⁻². Y³⁺ doping enhances ATW emissivity by inducing lattice distortion, which reduces symmetry and alters the dipole moment while boosting reflectivity in the visible and near-infrared (vis-NIR) regions by preserving the wide bandgap through the introduction of optically inert elements. This work synergistically integrates the advantages of high performance, low cost, and environmental friendliness, offering a highly promising ceramic material solution for large-scale radiative cooling applications.