In order to improve enzymolysis technology for preparation of angiotensin-converting enzyme(ACE) inhibiting peptides from oat-seed protein by alcalase protease, ultrasonic pretreatment of oat-seed protein solution was investigated. Based on the inhibitory activity of ACE and degree of hydrolysis (DH), the effects of ultrasonic power, pretreatment time and the on-off time ratio on ultrasonic pretreatment efficacy were studied. The results show that the optimum conditions of ultrasonic pretreatment are 500W of ultrasonic power, 20min of pretreatment time and 2s/2s of ultrasonic on-off time ratio. Under the optimum conditions, half inhibiting concentration (IC50) value of ACE decreases from 0.533mg/mL to 0.299mg/mL, and the time of enzymolysis reduces from 90min to 60min. Experiments also show that ultrasonic pretreatment parameters have made a significant impact on ACE inhibitory activity while no significant effect on the DH. It is verified that the increase of protein hydrophobicity caused by ultrasound is one of main reasons for significant improvement of hydrolysates ACE inhibitory activity. 