The aim of this study was to investigate the effect of sweeping frequency ultrasound pretreatment duration on the antioxidant activity of peptides prepared by β-lactoglobulin. The ultrasound pretreatment of different duration （0， 10， 20， 30， 60， 90 min） impacts on β-lactoglobulin （β-LG） hydrolysis and structural characteristics. The enzymatic hydrolysis characteristics of β-LG， such as the antioxidant activity， the surface hydrophobicity， the molecular weight distribution and the composition of amino acids of β-lactoglobulin hydrolysates were studied. The particle size， surface hydrophobicity and microstructure of β-LG after sweeping frequency ultrasound pretreatment were also determined. The results showed that sweeping frequency ultrasound pretreatment significantly improved the DPPH· scavenging rate， ABTS· scavenging rate and Fe2+ complexation rate. The antioxidant activity of β-LG hydrolysates increased significantly （P<0.05） with short time ultrasound pretreatment and decreased over a longer period （60，90 min）. After sweeping frequency ultrasound pretreatments， the surface hydrophobicity of enzymatic hydrolysates increased which indicated that more hydrophobic substances were released and polypeptides with molecular weight 200-2 000 u increased significantly which improved the antioxidant activity of β-LG hydrolysates. The amino acid profile analysis indicated sweeping frequency ultrasound pretreatment could increase the concentration of hydrophobic amino acids of the β-LG hydrolysates. Short time （< 30 min） ultrasound induced the particle size largely increased. After long-time sweeping frequency ultrasound （60， 90 min） treatment， the particle size of β-LG increased， which indicated that the exposed tryptophan residues aggregated into the protein.