In order to further develop the application of carmininc acid (CA) for fluorescence detection, oxidized carminic acid (OCA), which is water soluble and emits blue fluorescence, was synthesized in one step by an ethanol-assisted hydrothermal method using carminic acid as the raw material and potassium periodate as the oxidant. Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis showed that the structure of OCA contained conjugated aromatic rings and a large number of oxygen-containing functional groups, and transmission electron microscopy (TEM) and dynamic scattering light (DLS) indicated an average particle size of approximately 7.5 nm. Optical tests showed that the fluorescence emission of OCA appeared in the range of 380–620 nm, the maximum excitation wavelength was 336 nm, and the maximum emission wavelength was 445 nm. OCA showed excellent fluorescence stability in solutions at pH 1–9 and in different salt solutions containing anions. UV-visible absorption spectroscopy showed that its maximum absorption peak appeared at 225 nm. OCA had good selectivity for iron ions, which led to a significant decrease in its fluorescence intensity. The percentage of decrease in the fluorescence emission peak integrated area, (S₀–S)/S₀, showed a good linear relationship with the mass concentration of iron ions in the range of 11.2–56.0 mg/L, and the limit of detection was 2.5 mg/L. Applying an OCA-based fluorescent sensor to detect the content of iron ions in mushrooms, the recoveries of this method was 95.3%–110.4%, with relative standard deviation (RSD) between 2.3% and 4.6%, indicating that the fluorescence detection method is reliable.