Abstract: Oxidized carbon nanoparticles (OCNPs) with uniform size, good dispersion and excellent quenching performance were prepared by ultrasonic assisted nitric acid oxidation method. A fluorescence resonance energy transfer (FRET) system was constructed, with FAM as energy donors, OCNPs as energy acceptors and T-rich ssDNA as recognition molecules. The interaction between OCNPs and ssDNA brought the energy donor and acceptor in close proximity, leading to the quenching of fluorescence of FAM. After the addition of mercury ion (Hg²⁺), the distance between the donor and acceptor was increased and the fluorescence of FAM was recovered gradually, due to the formation of T-Hg²⁺-T structure. Based on the above-mentioned principle, an "off-to-on" fluorescence sensor was established for Hg²⁺ detection. The linear range of mercury ion detection was 0.1~10.0 nmol/L, and the detection limit was 0.06 nmol/L. The method showed good sensitivity and has been applied to the determination of Hg²⁺ in tap water and river water with a recovery of 97.4%~108.4%.