The effects of citrus flavonoids on the catalytic activity and kinetic parameters of trypsin were studied.The results showed that citrus flavonoids had obvious inhibitory effect on trypsin,and the inhibition showed concentration effect.At 37 ℃,the inhibitory rate of 0.66 mg/mL citrus flavonoids against trypsin was 29.9%.The inhibition belonged to the mixed inhibition type of competitive and non-competitive inhibition.The fluorescence spectra showed that citrus flavone (naringin) quenched the endogenous fluorescence of trypsin in a static quenching mode.At room temperature (293 K),the binding constant Ka was 1.1×106 L/mol,and the number of binding sites nwas 1.34.The thermodynamic parameters showed that hydrogen bond and van der Waals force played a dominant role in the interaction between trypsin and citrus flavonoids.