Mercury is a typical low-dose and high-toxic substance, which widely exists in the environment and water. It is transmitted and accumulated through the food chain, which is easy to cause harm to human body. Therefore, accurate and rapid monitoring of Hg2+ content is of great significance for ensuring food safety. At present, there are many techniques to detect mercury ions such as high performance liquid chromatography atomic fluorescence spectrometry (LC-AFS), inductive coupled plasmamass spectroscopy (ICP-MS), electrochemical methods and fluorescence analysis methods. Ratio fluorescence probe has dual emission fluorescence characteristics. The built-in calibration function can reduce the detection errors caused by probe concentration and various environmental factors, and effectively overcome the shortcomings of single emission fluorescence probe. A novel ratio fluorescent probe based on carbon quantum dots (CQDs) and copper nanoclusters (CuNCs) was proposed for the rapid detection of Hg2+ in crabs. The main research contents and results are as follows: (1) The preparation of CQDs-CuNCs composite system, CQDs were synthesized by microwave-mediated method with sucrose as carbon source and polyethylene glycol as passivator; CuNCs were synthesized by hydrothermal method using ascorbic acid as reductant and stabilizer, and then self-assembled into CQDs-CuNCs composite system. (2) Characterization of CQDs-CuNCs composite system. The CQDs-CuNCs composite system was characterized by high power transmission electron microscopy (HRTEM), ultraviolet-visible absorption spectroscopy (UV-Vis), fluorescence spectroscopy (FL) and Fourier transform infrared spectroscopy (FTIR). The results show that the CQDs-CuNCs ratio fluorescent probe with dual emission characteristics has been successfully synthesized. (3) The stability of CQDs-CuNCs composite system was tested. The stability of CQDs-CuNCs ratio probe was compared with that of traditional single-channel CuNCs probe. The results show that the migration of CQDs-CuNCs ratio probes is stronger and stabler than that of single-emitted CuNCs when the probe concentration drift and temperature fluctuation are measured. (4) The CQDs-CuNCs composite system detects Hg2+. When Hg2+ exists, the CuNCs in the composite system will agglomerate, but CQDs will not be affected, resulting in the fluorescence quenching of CuNCs at 443 nm and the fluorescence intensity of CQDs at 545 nm almost unchanged. Quantitative detection of Hg2+ was realized based on the relationship between the ratio of fluorescence intensity (I443 nm/I545 nm) and the concentration of Hg2+. In the standard sample detection, the quenching rate of CQDs-CuNCs composite system at I443 nm/I545 nm and single-emission CuNCs has a good linear relationship with Hg2+ (0.1～12 μmol·L-1), the correlation coefficients are 0.994 7 and 0.991 6, and the detection limits (3σ/S) are 2.83 and 3.62 nmol·L-1, respectively. In crab sample detection, the recoveries of CQDs-CuNCs ratio probe and single-emission CuNCs were 102.5%~105.4% and 104.2%~112.5%, respectively. The results show that the CQDs-CuNCs composite system has higher sensitivity and recovery to Hg2+ than single-emission CuNCs. The CQDs-CuNCs ratio fluorescent probe constructed in this study can be used for rapid and accurate detection of Hg2+ in food.