Abstract: This study investigated the limitations of pH differential method in the determination of anthocyanin liquid and the influence of free radical types on the evaluation of black soybean seed coat anthocyanin antioxidant activity. The results showed that pH of the buffer system significantly affected the anthocyanin content determined by the pH differential method (P< 0.05). In the range of pH 2.0-6.0, the anthocyanin content remained relatively stable before and after heat treatment, but in the range of pH 7.0-8.0, the content decreased sharply after heating. The inconsistent change trend might be caused by the structure transformation and degradation of anthocyanin at the higher pH. The pH significantly affected the scavenging rate of anthocyanins to ABTS and DPPH free radicals (P< 0.05), and the scavenging rate of anthocyanins under weak acid environment (pH 4.0-6.0) was significantly higher than that of strong acids and alkaline environments (P< 0.05). Moreover, the types of free radicals significantly affected the antioxidant activity trend of anthocyanin under different pH environments (P< 0.05). The pH of the buffer system and the types of free radicals significantly affected the determination of anthocyanin content and antioxidant activity, which may be related to the fact that DPPH is a nitrogen radical and ABTS is a nitrogen cation radical, while the salts may affect the absorbance determination by their solubilities.