In the present study, novel polymeric nanoparticles of ca. 55 nm in diameter with reversibly photoswitchable fluorescence properties were synthesized using a facile one-step miniemulsion polymerization, in which the donor of fluorescence resonance energy transfer (FRET), 4-methamino-9-allyl-1,8-naphthalimide (MANI), and the acceptor, spiropyran-linked methacrylate (SPMA), were covalently incorporated into a polymeric matrix during the polymerization process. The fluorescence emission of MANI dye in nanoparticles can be reversibly switched using the alternating irradiation of UV and visible light, which can induce the structural interconversion between the SP form and MC form of spiropyran moieties inside nanoparticles and thus reversibly switch on and switch off the FRET process. The prepared photoswitchable fluorescent polymer nanoparticles not only show a high load capacity of dyes, controllable amount and ratio of the two dyes, and tunable FRET efficiency but also exhibit higher spectral stability because of covalent linkage between dye molecules and the particle, relatively fast photoresponsibility, and better photoreversibility compared to some previously reported systems.