The invention belongs to the field of nano-materials, concretely relates to an oligonucleotide functionalized fluorescence nano-carbon dot, and a preparation method and a user thereof, and also relates to a fluorescence nano-carbon dot with citric acid as a carbon source and with the tail end modified by a carboxyl group, and a use thereof. The fluorescence nano-carbon dot and the oligonucleotide functionalized fluorescence nano-carbon dot have simple preparation methods, have adjustable photoluminescence properties and high fluorescence quantum yield, and have good application prospects in the fields of biological marking, positioning, tracing, sensing and targeting.