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BACKGROUND Flos Sophorae Immaturus (FSI) is rich in polyphenols and becomes a potential uric acid-lowering food. However, the processing of FSI is greatly restricted due to the heat sensitivity and low solubility of polyphenols. In this study, hydrothermal treatment, an effective strategy, was applied to FSI processing. The variation of xanthine oxidase (XO) inhibitory effect and polyphenols composition of FSI during hydrothermal treatment were recorded. RESULTS The XO inhibition rate of FSI increased from 32.42% to 89.00% after hydrothermal treatment at 220 °C for 30 min, as well as the total polyphenols (from 0.66 to 1.11 mg mL-1 ) and flavonoids (from 1.21 to 1.58 mg mL-1 ). However, high thermal temperature (> 160 °C) and extended thermal time (> 90 min) caused the degradation of polyphenols. Rutin, kaempferol-3-O-rutinoside, and narcissoside rapidly degraded and converted to quercetin, kaempferol, and isorhamnetin when the temperature exceeded 160 °C. The maximum yields of quercetin, kaempferol, and isorhamnetin were at 220 °C for 30 min, 90 min, and 90 min, respectively. Meanwhile, the conversion kinetics conformed to the first-order model. Interestingly, these newly formed polyphenols possessed better XO inhibitory effects than their derivatives with 3-O-rutinoside. CONCLUSION The polyphenols conversion during hydrothermal treatment was the main reason for enhancing the XO inhibitory activity. Therefore, hydrothermal treatment is an appropriate method for improving the XO inhibitory effect of FSI. This article is protected by copyright. All rights reserved.