The fluorescence reaction of cationic surface-active agents (CSAA) with eosin Y in aqueous solution was investigated. It was found that the fluorescence quenching of eosin Y appears when it reacts with the cation monomer of a CSAA and a new stronger fluorescence is obtained when the ion-associates react with the micellae of CSAA. The reaction has a very high sensitivity, its detection limits for different CSAA were around 6.6ng/mL to 12.0ng/mL, and can be applied to the trace determination of CSAA. This reaction also provides a new way to study the existence species of the CSAA and the fluorescence dye in aqueous solution. The fluorescence characteristics, the optimum conditions of the reaction system and the reaction mechanism were also discussed.