A novel fluorescent sensor based on natural quercetin and core-shell Ag@SiO2 nanoparticles for highly sensitive and selective detection of copper ions has been studied. The Ag@SiO2@Quercetin fluorescent sensor after binding to Cu2+ ions showed a quenching of fluorescence emission intensity. The sensor can be applied to the quantification of Cu2+ ions with a linear range of 3.0×10-7～4.8×10-6 mol·L-1 and a detection limit of 1.0×10-7 mol·L-1. The sensor showed high selectivity toward Cu2+. As a result, the proposed fluorescent nanosensor was successfully applied for determination of Cu2+ in water samples with good recovery.