A lot of peptides were synthesized on the basis of the sequence of αs1-casein. These tri-peptides with the characteristics of low toxicity and high biological activity will be applied to screen a new type of antihypertensive drug. Angiotensin-converting enzyme inhibitors (ACEI) with the better biological activity were screened and their structure-activity relationship was studied. The method of Fmoc solid-phase synthesis had been used to synthesize tri-peptides, and the principle, species, steps of which had been introduced respectively. HPLC assay was applied to screen the activity of ACEI by detecting the concentrate of hippuric acid, which can correspond with the inhibitory activity of tri-peptides. The results showed that the tri-peptides containing hydrophobic amino acid or praline had higher inhibition activity, and the same to the one that containing an aromatic amino acid in N-terminal.