Fluorescence spectroscopy in combination with circular dichroism and UV-Vis absorbance spectrum were employed to investigate the binding of thionine and bovine serum albumin system under physiological conditions. The experimental results showed that electrostatic interaction played a major role in the interaction, thionine could quench the fluorescence of BSA with a static quenching mechanism. The apparent binding constant/number of binding sites were 1. 155× 105 L·mol^-1/1. 835 for TH-BSA, respectively. The distance between BSA and TH was es...