The invention discloses a single-layer single-crystal graphene quantum dot. The particle size of the single-layer single-crystal graphene quantum dot ranges from 1 nm to 10 nm. The single-layer single-crystal graphene quantum dot is of a single-layer single-crystal structure. The single-crystal structure is a typical hexagonal mesh-like fringe structure. The invention further discloses a preparation method of the single-layer single-crystal graphene quantum dot. According to the method, polycyclic aromatic hydrocarbon serves as a carbon source, the single-layer single-crystal graphene quantum dot is prepared through a liquid phase stripping method, the obtained quantum dot can emit bright fluorescence, and the emission wavelength can be adjusted and controlled through a carbon source precursor and the pH value of a graphene quantum dot solution.