A fluorescent method was developed for the quantitative analysis of iodine by synthesizing BSA-protected AuNCs（AuNCs@BSA） as fluorescent probe.Under the optimum conditions,the decrease of the fluorescent intensity of AuNCs@BSA had a good linear relationship with iodine concentration in the range of 2.0 nmol/L-35 μmol/L,and the detection limit for iodine was 1.8 nmol/L.The proposed method was used to detect iodine in water,and the results were consistent with those obtained by ICP-MS method,which indicated that this method was a rapid and reliable technique for detection of iodine in real samples.The quenching mechanism of iodine with AuNCs@BSA was studied by the modified Stern-Volmer equation and thermodynamic calculation,and CD and IR results revealed that the secondary-structure change of the capping ligand BSA played an important role in the fluorecent quenching of AuNCs@BSA.In conclusion,the accurate and full data would make a significant contribution to understanding of the action mechanism and relationship between inorganic small molecules and protein protected metal nanoclusters.