Achieving polymer nanocomposites with improved elasticity and stable mechanical and fluorescence properties remains challenging due to the aggregation tendency of fluorescent carbon nanomaterials, particularly for a polymer with no functional groups. To alleviate the aforementioned issues, the “grafting from” method can be employed via covalent bonding design. Herein, we exploit the “grafting from” method coupled with reversible fragment-added chain transfer polymerization to construct soft polyisoprene (PI) chains onto a photostable nitrogen, sulfur, and phosphorus-doped carbon quantum dot (NSP–CQD) surface for the preparation of flexible fluorescent NSP–CQDs-grafted-polyisoprene (NSP–CQDs-g-PI x ) nanocomposite films. By tuning the molecular weight of PI in NSP–CQDs-g-PI x (x = 8, 13, and 18 kg mol–1), the elastic and mechanical properties can be tailored. Upon an intense tensile cyclic test, the NSP–CQDs-g-PI18K generates remarkable elasticity, indicated by the absence of permanent deformation or residual strain even after multiple loading–unloading tests up to 50 times. Moreover, compared with the blending method that tends to cause reduced photoluminescence properties due to the NSP–CQD agglomeration, the grafting method intelligibly maintains the fluorescence property. This study concludes that “grafting from” is a feasible method for fabricating CQD-loaded composites, which have great potential to meet the demand for advanced stretchable and flexible organic electronic devices.