In this work, we synthesized blue-fluorescent nitrogen-doped carbon quantum dots (N-CQDs) via a facile, economical, and environmentally friendly one-pot synthesis, using citric acid as the carbon source and polyethyleneimine (PEI) as the nitrogen dopant. The as-prepared N-CQDs exhibited uniform size distribution, with an average diameter of approximately 3 nm and a quantum yield of up to 23.6%. Based on the mechanism of HClO-triggered static fluorescence quenching and oxidation of surface amine groups on the N-CQDs, we established a quantitative detection platform for hypochlorous acid (HClO). The proposed method demonstrated a linear response over the concentration range of 0–40 μmol/L, with a detection limit as low as 0.17 μmol/L. It also featured a rapid response time (within 2 min), high selectivity, and strong anti-interference capability against various common species, including Cl⁻, H₂O₂, NO₂⁻, NO₃⁻, TBHP, TBO•, Br⁻, I⁻, S²⁻, F⁻, O²⁻ and HO•. Furthermore, the probe was successfully applied to detect HClO in real-world samples such as river water and beer. Owing to its outstanding photostability and low toxicity, it proved highly effective for monitoring intracellular HClO in living cells.