The　potential　optoelectronic　applications　of　perovskite　nanocrystals　(NCs)　are　primarily　limited　by　major　material　instability　arising　from　the　ionic　nature　of　the　NC　lattice.　Herein,　we　introduce　a　facile　and　effective　strategy　to　prepare　extremely　stable　CsPbX3　NC　polymer　composites.　NC　surfaces　are　passivated　with　reactive　methacrylic　acid　(MA)　ligands,　resulting　in　the　formation　of　homogeneous　nanocubes　(abbreviated　as　MA-NCs)　with　a　size　of　14-17　nm　and　a　photoluminescence　quantum　yield　above　80%.　The　free　double　bonds　on　the　surface　then　serve　as　chemically　addressable　synthetic　handles,　enabling　UV-induced　radical　polymerization.　Critically,　a　bromide-rich　environment　is　developed　to　prevent　NC　sintering.　The　composites　obtained　from　copolymerizing　MA-NCs　with　hydrophobic　methyl　methacrylate　and　methacrylisobutyl　polyhedral　oligomeric　silsesquioxane　monomers　exhibit　enhanced　properties　compared　to　previously　reported　encapsulated　NCs,　including　higher　quantum　yields,　remarkable　chemical　stability　toward　water,　and　much　enhanced　thermal　stability.　The　good　solubility　of　the　composite　in　organic　solvent　further　enables　its　use　as　a　solution-processable　luminescent　ink,　used　here　for　fabrication　of　white　light-emitting　diodes　with　high　luminous　efficiency　and　excellent　color-rendering　index.　The　resulting　fluorescent　and　stable　NC　ink　opens　the　door　to　potential　scalable　and　robust　optoelectronic　applications.