Collagen peptides were extracted from grass carp scales by ultrasound-assisted enzymatic hydrolysis to explore the effects of ultrasonic power (0~600 W) and duration (0~40 min) on the yield, degree of hydrolysis, and physicochemical properties of the collagen peptides obtained. Alkaline proteinase, compound proteinase, and neutral protease were used during single-enzyme enzymatic hydrolysis. Ultrasound treatment significantly influenced the yield. However, the degree of hydrolysis and nitrogen yield initially increased and then decreased upon ultrasound application. In particular, 300 W ultrasound treatment for 20 min during alkaline protease-mediated hydrolysis increased the degree of hydrolysis from 5.37% to 7.27%. In addition, a 10 min 300 W ultrasound treatment during step-wise enzymatic hydrolysis with alkaline protease and flavourzyme, increased the degree of hydrolysis from 9.26% to 11.05%. Ultrasound also influences the molecular weight distribution and amino acid composition of the products. The molecular weight of the products was mainly within 500 and 1000 u. The content of collagen peptides extracted by single-enzyme enzymatic hydrolysis increased from 24.26% to 33.99%, while the content of collagen peptides prepared by step-by-step enzymolysis increased from 31.99% to 39.28%. Meanwhile, the amino acid content of products from single-enzyme enzymatic hydrolysis and step-by-step enzymolysis increased from 66.30 g/100 g to 73.75 g/100 g and 66.05 g/100 g to 70.70 g/100 g, respectively. Furthermore, ultrasound treatment also significantly affected the emulsification performance, foaming capability, and foam stability of the products. To sum up, single-enzyme hydrolysis can be optimized by 20 min 300 W ultrasound treatment during alkaline protease hydrolysis, while step-by-step enzymolysis can be optimized by 10 min 300 W ultrasound treatment during alkaline protease hydrolysis and another 10 min 300 W treatment during flavourzyme hydrolysis.