We report a one-pot route for the synthesis of CsPbBr3 perovskite nanocrystals (PNCs) in styrene to form a glue-like polystyrene (PS) pre-polymer incorporating mono-dispersed PNCs. The pre-polymer enables solvent- and initiator-free fabricating and patterning PNC-PS light down-conversion films for liquid crystal display application. The mechanistic study reveals that the styrene molecules adsorbed on the PNC surface undergo self-initiated polymerization in the pre-polymerization process, forming stable surface capsulation over the PNCs. The PNC-PS pre-polymer and composite film display high photoluminescent quantum yield (PLQY) and resistance to air, light irradiation and water. The micropatterned PNC-PS film with a period of 1000 nm was fabricated through imprinting of the pre-polymer. The micropatterned thin film displays an enlarged viewing angle, improved light distribution and PLQY of >90%. The backlight employing the PNC-PS film displays bright green color and a wide color gamut of >120% NTSC. This solvent-free and one-pot strategy could find promising potential in the development of diverse luminescent nanocomposites to meet the requirements of micro/nano-manufacturing and high performance display application.