To study the effect of ultrasound treatment on structure and functional properties of casein,this article studied the changes of particle size,solubility,foaming capacity,emulsifying activity,secondary structures,flexibility and molecular weight of casein after ultrasound treatment. The results indicated that the solubility of casein was gradually increased with the increase of ultrasonic power and time. When ultrasonic power was 480W,the foaming capacity of casein solution was the highest 17. 03% higher than that of untreated; and emulsifying activity at 960W was 31. 78% higher than that of untreated. Setting the ultrasonic power up to 240W,the highest foaming capacity was obtained at 25 min,and highest emulsifying activity was obtained at 15 min,both increased by 22. 65% and 18. 88% than untreated ones. The particle size of casein decreased and the particle distributed more homogeneous after an appropriate ultrasound treatment. At the same time,the treatment of ultrasound may also increase the flexibility of casein. The content of α-helix was decreased significantly after ultrasound treatment,when the casein solution was at ultrasound 240W for 15 min,the content of α-helix decreased by 4. 80%. However,ultrasound treatment had no effect on the molecular weight of casein,which indicated that ultrasound did not break the peptide bonds.