Iodine is one of essential trace elements. Deficiency of iodine and excess intake of iodine both can lead to thyroid diseases. Therefore, it is of great significance to develop a highly sensitive and selective method for the detection of iodine ions. Traditional analytical methods for iodine ions are usually involved in complex sample pretreatment and precision instruments, which are unfavorable for in-situ rapid detection. Fluorescent methods have been attracted great attentions due to their high sensitivity, high selectivity and easy operation. However, the present probes for iodine ions usually need complex organic syntheses and iodine ions are detected by means of coordination with heavy metal ions, which are unfavorable to promote the use of these methods. Fluorescent graphitic carbon nitride (g-C3N4) nanomaterial has attracted more attention due to the advatages of low-cost, easy preparation, high quantum yield, excellent photostability, and low toxicity. Furthermore, these nanomaterials can avoid complex synthesis for organic fluorophores or potential damage to environment for metal semiconductor quantum dots. These features make g-C3N4 nanomaterial an emerging fluorescent probe for the detection of metal ions. Recently, it was reported that Hg2+ ions could selectively and sensitively quench the fluorescence of g-C3N4 quantum dots (QDs). The addition of iodine ions could abstract the bound Hg2+ ions to form HgI2 complexes and restored the fluorescence of g-C3N4 QDs. Therefore, the fluorescent sensor for iodine ions could be developed. However, heavy metal ions (Hg2+) are also involved in this method, which limits its application. In this work, water-soluble g-C3N4 QDs with high fluorescence emission were prepared by using chemical oxidation of bulk g-C3N4 in nitric acid and hydro-thermal treatment. The maximal emission wavelength of g-C3N4 QDs located at 368 nm and did not change with the excitation wavelength, which indicates the size of g-C3N4 QDs is relatively uniform. There was a strong absorption peak at around 220 nm for iodine ions, which was overlapped with the fluorescent excitation spectrum of g-C3N4 QDs. On addition of iodine ions, the fluorescence of g-C3N4 QDs was quenched due to the inner filter effect. Therefore, a sensitive and selective fluorescent sensor for iodine ions was developed. Under the optimal conditions, there was a linear relationship between the fluorescence quenching (ΔF) of g-C3N4 QDs and the concentration (X, μmol·L-1) of iodine ions over the range of 10～400 μmol·L-1. The linear equation is ΔF=0.325 79X+6.039 05 (R2=0.999 5). The limit of detection is 5.0 μmol·L-1. The detection of iodine ions can be completed by “mixing and testing” without the need of coordination with heavy metal ions. Thus, this sensor is rapid, environment-friendly, simple and convenient.