Being a common hazardous waste, 4-nitrophenol (4-NP) has caused a serious threat to humans and environment. Therefore, rapid and selective detection of 4-NP, especially using a simple and portable instrument, is highly desired for human health and environmental monitoring. Herein, we develop a novel pH-mediated reversible fluorescence nanoswitch for selectively detecting 4-NP by using water-soluble fluorescent polymer carbon dots (PCDs) as a probe. The fluorescence of PCDs can be quenched by 4-NP via inner filter effect (IFE) because its excitation spectrum well overlaps with the absorption spectrum of 4-NP under alkaline condition. However, an obvious blue shift of the absorption peak of 4-NP occurs under acidic condition, causing the fluorescence recovery of PCDs due to the disappearance of IFE. On the basis of this principle, a pH-mediated reversible fluorescence nanoswitch was constructed and a broad linear range was obtained from 0.5 to 60 mu M with a detection limit of 0.26 mu M for 4-NP. Furthermore, this approach was successfully applied to detect 4-NP in real water samples and a portable polyamide film-based sensor was developed for visual detection of 4-NP, which offers a promising platform for the detection of 4-NP in on-site and resource-poor settings.