In the human body, iodine is the key raw material for thyroid hormone. Moreover, thyroid hormone had a critical role in human growth and development and regulated metabolism, so iodine was played a vital role in the normal operation of the human body. The iodine was mainly in the form of iodinated amino acids presented in the thyroid gland. Furthermore, many methods were used to detect I^- with high precision and accuracy. However, most of the approaches lie in complicated operations and high detection cost. Therefore, it is of great significance to develop a method for detecting I^- with low cost, fast, accuracy and simple operation. In this study, a simple and convenient fluorescence “Off-On” detection method was used for I^- testing in solution. The carbon nanodots (CDs) were synthesized by a simple hydrothermal method, which citric acid and ethane diamine were used as the carbon source and the nitrogen source. The optical properties of the CDs were characterized by a fluorescence spectrometer and an ultraviolet-visible spectrometer, which emitted bright blue light at an excitation wavelength of 350 nm. Additionally, the groups on the surface of the CDs were characterized by the Fourier-infrared spectroscopy and high resolution ultraviolet photoelectron spectroscopy. The fluorescence of the prepared CDs can be pre-quenched by Hg²⁺. And the fluorescence quenching may be due to the non-radiative electron-transfer from the excited states to the d orbital of Hg²⁺. Another possible explanation is attributed to the Hg²⁺ ions combine with a large amount of nitrogen on the surface of the CDs to form a non-fluorescent complex, which leads to fluorescence quenching of CDs. The optimal concentration condition of Hg²⁺ was selected by analyzing the fluorescence spectrum of CDs solution with different concentration Hg²⁺. Once they meet I^- in the solution, Hg²⁺ on the surface of the CDs will be released due to the strong interaction between Hg²⁺ and I^-, and the fluorescence of CDs can be restored. Thus, I^- in the solution can be detectable by this fluorescent “Off-On” method, and the detection range is 5.0~75 μmol·L^-1 with the detection limit of 0.25 μmol·L^-1. Finally, the selectivity and anti-interference ability of Hg²⁺@CDs fluorescence sensing system were analyzed by fluorescence spectrum. And the results showed that the fluorescence sensing system had high selectivity to I^- and good ion anti-interference performance.