The interaction mechanisms between isoflavones (Genistein, 3’,4’,7-trihydroxyisoflavone and Biochanin A) and different isomers of human serum albumin (HSA) were investigated by fluorescence spectroscopy. Various parameters (quenching rate constants, binding constants and number of binding sites) of isoflavones-human serum albumin complexes were calculated. The results showed that the isoflavones have only one binding site on human serum albumin, located at the Site I, and the binding constants were between 0.17×105 and 1.20×105 L·mol-1. Fluorescence enhancement experiments showed that the fluorescence intensities of the drugs significantly increased after interacting with HSA, indicating that the combination of drug and HSA had occurred. The binding mechanisms between three isoflavones and HSA were discussed.