In the present study, an easy one-pot solid-phase synthesis strategy is adopted to obtain nitrogen-doped graphene quantum dots with a high quantum yield using citric acid and urea as carbon and nitrogen sources, respectively; based on this synthesis, we report a turn-on fluorescent sensor for a selective detection of cysteine on recovered Hg(II)-modulated nitrogen-doped grapheme quantum dot fluorescence. Compared with previous reports [20–22], this method showed a lower detection limit, but is simpler, faster, inexpensive, and more efficient than the other methods. The high affinity of cysteine to Hg(II) enables the dissociation of the ion from the surface of nitrogen-doped graphene quantum dots, thereby forming a stable complex with cysteine in the solution, and recovering the fluorescence of the nitrogen-doped graphene quantum dots. The developed nitrogen-doped grapheme quantum dot-based photoluminescent sensor was successfullyapplied in the detection of cysteine in real sample.