The effects of ultrasonic pretreatment on the enzymolysis thermodynamics and physicochemical properties of whey protein, and the mechanisms behind those effects, were investigated. Changes in the angiotensin-I converting enzyme (ACE) inhibitory and immunomodulatory activities of whey protein hydrolysates after ultrasonic pretreatment were also determined. Results showed that the ultrasonicated whey protein had a higher in degree of hydrolysis than the non-sonicated protein. After the pretreatment, the activation energy (Ea), enthalpy of activation (ΔH) and entropy of activation (ΔS) of whey protein enzymolysis were decreased by 15.9%, 16.8%, and 16.4%, respectively. There was no significant change in free energy (ΔG) (P > 0.05). Physicochemical analysis revealed that ultrasound had induced unfolding of the whey protein, resulting in a 43.7% increase in its surface free sulfhydryl content and a 62.6% increase in surface hydrophobicity. Ultrasound significantly decreased the protein’s α-helical content and significantly increased its β-sheets and β-turns (P < 0.05). The ACE inhibitory and immunomodulatory activities of the whey protein hydrolysates were significantly increased by ultrasonic pretreatment (P < 0.05). These results suggest that ultrasound can be applied to enhance whey protein enzymolysis for the generation of novel bioactive peptides that can be used as drug or functional food ingredient.