Abstract: The interaction between sodium caseinate and (-)-epigallocatechin-3-gallate (EGCG) and the antioxidant activity of their complexes as influenced by ultrasound pretreatment were studied by a fluorescence spectrophotometer and an ultraviolet spectrophotometer, and its mechanism was also investigated base on the changes in protein structure. The results showed that the quenching of the endogenous fluorescence of sodium casein induced by EGCG was a static nature. With ultrasound pretreatment, the apparent binding constant between sodium caseinate and EGCG increased from 9.51×10³ L·(mol/L)⁻¹ to 7.48×10⁴ L·(mol/L)⁻¹ and the number of binding sites between sodium caseinate and EGCG increased from 0.871 to 1.067, indicating an increase of 7.87 times and 22.50%, respectively. The results revealed that ultrasound pretreatment could enhance the binding capacity of sodium caseinate for EGCG. Compared with sodium caseinate, ultrasound pretreated sodium caseinate showed higher random coil content, fluorescence intensity and surface hydrophobicity. These ultrasound-induced conformational changes promoted the binding between sodium casein and EGCG. Complexing with sodium caseinate could inhibit the degradation of EGCG and improve its antioxidant stability, and better protection was obtained by complexing EGCG with ultrasound pretreated sodium caseinate.