Quantum-confined CsPbBr₃ nanocrystals are promising cyan/blue-emitting materials with exceptional potential for advanced lighting and display technologies. However, their practical application is often limited by low luminescent efficiency, undesirable color purity and instability stemming from inadequate size uniformity, and high surface defect density. Here, we propose a post-synthetic chemical cutting and in-situ encapsulation approach using 3-aminopropyltriethoxysilane (APTES) combined with polymethylhydrosiloxane (PMHS) to synthesize and stabilize CsPbBr₃ nanocrystals with significant quantum confinement. APTES can function as a chemical scissor efficiently cutting the green emitting CsPbBr₃ (9.8 nm) to smaller counterparts (3.9–7.9 nm), while also passivating surface defects through A-site doping. Crosslinking between APTES and PMHS prevents phase transformation during synthesis and forms a polymeric network that encapsulates and separates the quantum confined CsPbBr₃. The resulting composites exhibit tunable emission from 517 to 461 nm, a narrow linewidth of approximately 15 nm, and quantum yields over 80%. Moreover, incorporating one CsPbBr₃ composite into a white light-emitting diode to fill the “cyan gap” significantly enhances the color rendering index from 77.7 to 86.4. This work provides an effective strategy for developing bright and stable quantum-confined CsPbBr₃ for advanced lighting applications.