The operational longevity and performance stability of hygroscopic scintillators (e.g., CsI:Na) are critically limited by moisture‐induced degradation in radiation detection applications. To address this challenge, we propose an innovative thin film encapsulation strategy utilizing 100‐nm‐thick high‐quality amorphous Al₂O₃ film deposited at room temperature via filtered cathodic vacuum arc (FCVA) technology. Advanced plasma diagnostics reveal that pulsed magnetic field confinement enables precise regulation of plasma properties, facilitating the deposition of high‐purity dense amorphous Al₂O₃ film. The film exhibits outstanding optical transmittance (88.35% in the visible range) and an ultralow water vapor transmission rate of 5.73 × 10⁻⁴ g/m²/day under accelerated aging conditions (85℃, 85% RH). Remarkably, the encapsulated CsI:Na scintillator demonstrates a 14.53% enhancement in luminescence efficiency, attributed to effective refractive index matching that minimizes Fresnel reflection losses at the Al₂O₃/CsI:Na interface. Most importantly, the encapsulated scintillator retains over 98% of its initial luminescence after 20 h of exposure to harsh conditions (45℃, 85% RH), achieving a 40‐fold extension in operational lifespan compared to unprotected counterparts. This FCVA‐confined thin‐film encapsulation approach not only provides a robust and scalable solution for safeguarding hygroscopic scintillators but also establishes a versatile platform for developing next‐generation, environmentally stable radiation detection systems.