In summary, a fast neutralization heat strategy was employed herein to synthesize N-CDs with a specific “turn-on” fluorescence behavior for Pb²⁺ sensing. This established sensing platform could perform a linear fluorescent response towards Pb²⁺ in the concentration range of 0–50 μM. More significantly, both an excellent detection sensitivity (LOD of 0.92 μM) and selectivity were achieved for Pb²⁺ detection. Accordingly, a suppressed PET process caused by Pb²⁺ was proposed for the unique “turn-on” sensing mechanism of N-CDs, which was also proved by DFT simulation and experimental analysis. This work can inspire a broad development of nano chem-sensors in the field of environmental analysis and monitoring.