The application of fluorescent nanoparticles to the detection of inorganic ions and organic compounds has been attracted wide attention recently. In this paper, an "off-on" method for highly sensitive and selective detection of Hg and l-cysteine (l-Cys) or Hg and I using home-made nitrogen-doped carbon quantum dots (N-CQDs) as fluorescent probe was reported. The N-CQDs with a fluorescence quantum yield of 42.2% were prepared using tartaric acid, citric acid and ethanediamine as the precursors in the oleic acid media. The fluorescence of the obtained N-CQDs could be quenched selectively and sensitively by the addition of Hg (turn-off) with a detection limit of 83.5nM. When l-Cys or I was added into the N-CQDs-Hg system, the fluorescence was recovered effectively (turn-on). This process could be used to the detection of l-Cys or I with a detection limit of 45.8 and 92.3nM, respectively.