Mercury is a heavy metal element with significant accumulation effect and genotoxicity, which is extremely harmful to human health and the ecological environment. In China, the water environment is facing serious problems of mercury pollution. Developing rapid, efficient, and economical method for mercury ion detection can effectively promote the source control of mercury pollution in the aquatic environment. This study innovatively proposed a method, namely thiamine-fluorescence excitation-emission matrix (EEM), to monitor mercury ion in the water environment. Results showed that the position and number of fluorescence peaks of thiamine significantly changed after it happed redox reaction with mercury ion, which could be used as a characteristic signal for detecting mercury ions in water. In addition, when using this method to detect mercury ions in water, it was suggested that the concentration of thiamine should not be too high, and the reaction system should be kept in the alkaline environment. The reaction temperature and reaction time could be further optimized by the first-order kinetic model to reduce the detection cost and improve the detection effectiveness. Under the specific detection conditions (thiamine concentration 10 μmol·L-1, pH 9.7, reaction time 120 min, temperature 20 ℃), the linear detection range of mercury ion concentration was suggested to 4～15 μmol·L-1. The thiamine-EEM method owns outstanding advantages and good practical application values compared with the traditional method of mercury ion monitoring in water, which can effectively help the pollution source supervision of mercury in water environment and greatly improve the efficiency of environmental law enforcement.