The preparation of food-derived peptides with angiotensin transferase (ACE)-inhibitory activity from yak milk casein was explored. One strain with the potential to produce ACE-inhibitory peptides was screened from sour yak milk using the skimmed milk plate and Lowry methods and was combined with five proteases to hydrolyze yak milk casein. Hydrolysis degree, ACE inhibition rate, peptide concentration, and pH were used as indices to determine the optimal proteases, and the hydrolysis process was optimized via bacteria-enzyme cofermentation and hydrolysis using an orthogonal test. An ACE-inhibitory peptide from yak milk casein was isolated and purified using ultrafiltration and dextran gel G-15, and its amino acid sequence was determined using liquid chromatography tandem mass spectrometry (LC-MS/MS). The effect of temperature, pH, and a buffer salt system on its stability was determined. The potential strain Lactobacillus fermentum 187 with 43.29% ACE inhibition was screened, and the optimal protease was alkaline protease, with optimal enzyme preparation process parameters including an enzyme-to-substrate ratio of 767 U/g, 5% lactobacillus inoculation, an enzyme digestion time of 10 h, an enzyme digestion temperature of 37 ℃, and a substrate pH of 7.0. The G2 peptide fraction (IC50=0.28 mg/mL) was separated via ultrafiltration and dextran gel G-15 and exhibited a promising antihypertensive effect; the G2 peptide fraction also has good thermal, acid-base, and salt stability. In addition, a total of nine peptide sequences were identified by LC-MS/MS. The results of this study provide a theoretical reference for the development and utilization of bacterial and enzymatic co-fermentation and the hydrolysis of yak casein to produce antihypertensive peptides.