<p>To address the critical challenge of suppressing glass-phosphor interface reactions in high-melting-point phosphor-in-glass (PiG) for high-power laser illumination, this study fabricates a Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) phosphor-in-silica glass (YAG:Ce-PiSG) with high quantum efficiency and stability by regulating alkali metal oxides. Incorporation of Cs₂O notably inhibits SiO₂-YAG:Ce reactions, maintaining the PiSG’s internal quantum efficiency (IQE) at 97.7% of pure YAG:Ce (88.3% even after 1400 °C calcination for 2 h), while smaller alkali ions (Li⁺, Na⁺) accelerate YAG:Ce decomposition. Cs₂CO₃ acts as a flux to promote the melting of nano-SiO₂, thereby facilitating the formation of a dense, transparent glass matrix. Owing to the large ionic radius of Cs⁺ and its weak interaction with oxygen ions, the ready generation of non-bridging oxygen (NBO) is suppressed, consequently enabling the formation of a complete silica glass network that prevents alkali metal ions from etching the YAG:Ce phosphor. Leveraging the mixed alkali effect (10% Li₂O + 5% Cs₂O), the PiSG retains excellent luminescence and resists 200 °C hydrothermal treatment for 10 h. The PiG film-sapphire device delivers 3080 lm luminous flux and 213 lm W⁻¹ efficiency under blue laser excitation. These findings demonstrate that the developed YAG:Ce-PiSG serves as a highly promising color-conversion material for high-performance laser illumination.</p>