Hydrothermal treatment of m‐phenylenediamine and grape seed powder has been adopted to synthesize nitrogen‐doped carbon quantum dots (N‐CQDs). The prepared N‐CQDs possessed outstanding optical properties and high quantum yield. Based on the combined effect of static quenching effect and inner filter effect of permanganate (MnO4−) to N‐CQDs and the redox reaction that occurred between MnO4− and l‐ascorbic acid (l‐AA), an ‘off–on’ fluorescence strategy with N‐CQDs has been proposed for the detection of MnO4− and l‐AA. The proposed fluorescent probe was fast, sensitive and selective to MnO4− and l‐AA under mild conditions. In addition, the satisfactory results of the proposed strategy for the detection of MnO4− and l‐AA in real samples indicated its practicability.