The invention provides a fluorescent carbon dot for nuclear staining. The fluorescent carbon dot is prepared from the following steps: 1) dissolving folic acid and m-phenylenediamine in water, and carrying out heating for a reaction; and 2) after the reaction, performing centrifuging and silica gel column chromatography, collecting a yellow-green fluorescent part, transferring the yellow-green fluorescent part into water after rotary evaporation and then carrying out lyophilization. The carbon dot prepared in the invention emits green light under ultraviolet excitation, has large Stokes shift,is consistent with the existing commercial 405 laser at the optimal excitation position, and can be well applied to a laser confocal microscope.