As a new member of carbon nanomaterials, carbon quantum dots (CDs) with many advantages such as high optical stability, low toxicity, superior water solubility, diverse raw materials and preparation approaches, have shown application prospects in the fields of analytical detection, biomarker, photocatalytic degradation and environmental monitoring widely. The investigation on CDs has attracted significant interest. In general, the exceeding content of Fe³⁺ ion in water would be harmful to daily drinking and industrial production. It is of great significance to determine the content of Fe³⁺ in water accurately and quickly. At present, some techniques are used for the detection of Fe³⁺ ions, which include voltammetry, fluorescence spectrum, electrochemical and flame atomic absorption spectrometry. The fluorescence spectrometry has shown the merits of fast response and facile process, which makes it much better than other ways. In this paper, the CDs with bluish-green emission were prepared by hydrothermal treatment of persimmon leaves. The X-ray diffraction, high resolution transmission electron microscopy, Fourier transform infrared spectroscopy, ultraviolet visible absorption spectroscopy and fluorescence spectroscopy were used to characterize CDs’ structure, morphology, and spectroscopic properties. The CDs exhibit uniform spherical particles with an average diameter of 5.9 nm and abundant oxygen-containing functional groups on the surface. The UV absorption at 277 nm should be attributed to then→π^* transition of the C=O group. CDs’ emission wavelength and intensity are closely dependent on the excitation wavelength. Excited with 410 nm long, the emission maxima are 498 nm and it shows the strongest intensity. The fluorescence lifetime is about 4.59 ns. Moreover, the as-prepared CDs show high selectivity for Fe³⁺ ion compared to other metal ions, which can be used as a fluorescent probe to detect the trace concentration of Fe³⁺ in water. The dependence of fluorescence quenching rate F₀/F with Fe³⁺ concentration has a good linear relationship (R²=0.992), and the quenching constant, and detection limit value is 8.84×10³ L·mol^-1 and 0.21 μmol·L^-1, respectively. The detection limit value of 0.21 μmol·L^-1 is smaller than those reported in recent literature. Consequently, this work provides a preparation process with natural raw materials, simple operation and low-cost, and develops a new pathway for the fluorescence detection of trace metal iron ions in water.