Objective:To determine dissociation constants of THSG by UV spectrophotometry. Method: THSG was dissolved in Britton-Robinson buffer solutions of different pH values and the absorbance was calculated according to the suitable wavelenths. Results:The pKa1 value was found to be 790 in Britton-Robinson buffer solutions using wavelengths 319,325 and 330nm for the determination,and the pKa2 value was found to be 1041 in Britton-Robinson buffer solutions using wavelengths 330,340 and 350nm for the determination. Conclusion: The present method is convenient and practicable.This study lay the foundation for the investigations of absorption,distribution and metabolism of the durg.