A rapid method for the determination of ethanol content is proposed and tested. A fluorescence detecting system, with a multimode fiber (MMF) sensing head, is employed. Rhodamine 6G (R6G) is applied as the fluorescent indicator. In the R6G aqueous solution, the molecules aggregate at high concentration, causing fluorescence quenching. Nevertheless, aggregation and quenching rarely occur in ethanol. Taking an ethanol and water mixture as the solvent, the photoluminescence (PL) intensity reflects the aggregation degree and the ethanol content. Based on this phenomenon, the contents of the ethanol-water mixture were measured through PL intensity detection. A limit of detection (LOD) at ∼0.1 vol% level was obtained in the range of 0-100%. Commercial Chinese baijiu and rubbing alcohol were tested and the results obtained were consistent with the label values. The detecting system is compact and of low-cost, and the detecting method is rapid, accurate and repeatable. There is good prospect of applications for the determination of ethanol content on-site.