Abstract: Object: To prepare Pu-erh tea nano-selenium doped carbon quantum dots (PT-Se-CQDs) for the rapid detection of Fe³⁺ in the water system and to profile their characteristics. Method: In this study, PT-Se-CQDs and elemental selenium were prepared simultaneously in a water-bath by optimizing the reaction temperature and time. The ultraviolet-visible absorption and fluorescent intensity of PT-Se-CQDs were subsequently analyzed by the ultraviolet-visible absorption spectroscopy and fluorescence spectroscopy. And their morphology, elemental composition, and structural characteristics were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction, respectively. On this basis, a novel fluorescence sensor for the detection of Fe³⁺ in the aqueous system was constructed using PT-Se-CQDs. Result: PT-Se-CQDs in a spherical shape with a quantum yield of 3.41%, an average particle size of about 3.1 nm as well as elemental selenium were successfully prepared simultaneously via the reaction in a boiling water bath at 100 °C for 10 h. In addition, a strong static fluorescence quenching effect on PT-Se-CQDs was observed in the presence of Fe³⁺. Accordingly, Fe³⁺ in the range of 0~300 μmol/L was successfully detected using PT-Se-CQDs as a fluorescence sensor with a good linear relationship between the concentration of Fe³⁺ and the ratio of fluorescence intensity (F/F₀) of PT-Se-CQDs (R²>0.99) and a limit of detection of 0.2621 μmol/L. When this method was applied to detect Fe³⁺ in real water samples, satisfactory standard recovery rates of Fe³⁺ in pure water and mineral water of 90.93%~104.56% and 84.53%~113.90% with the RSD less than 8.15% and 4.00% were obtained, respectively. Conclusion: The preparation of PT-Se-CQDs with high selectivity and sensitivity to Fe³⁺ and their application as a new fluorescence sensor for the detection of Fe³⁺ in aqueous systems with simple operation and fast response were explored in the present study.