The interactions between alizarin red S(ARS) and bovine serum albumin(BSA) at various pH values were investigated by spectrometry and molecular docking technique.At pH 4.0,the natural compact conformation of BSA was unfolded gradually,which led to the exposure of hydrophobic cavity of domain ⅡA in BSA and decreased the interaction between BSA and ARS.Consequently,the binding constant Kb is only 3.39×104 L?mol-1.However,when the pH value was higher than the isoelectric point(pI 4.8 )of BSA,the interaction between BSA and ARS was enhanced and Kb increased to 3.16×106 L?mol-1 at pH 7.0.Moreover,the effect of surface charges on the interaction of BSA with ARS was not obvious due to the strong hydrogen bonds and van der Waals,forces,which was in accord with the results of molecular docking.