Eight sansalvamide A peptide analogues with 4'-fluoride, 4'-chloride, 4'-bromide, 4'-iodide, 4'-hydroxyl, and 4'-methoxyphenylalanine moieties were synthesized to investigate the effect of para-substitutions on the phenylalanine moiety of sansalvamide A peptide on its antitumor activity. Their cytotoxicity was evaluated against HCT-116, MDA-MB-231, HT-29, HCT-15, K562, HeLa, and A549 cell lines. The 4'-methoxyphenylalanine analog (10) was found to be a potent and selective antitumor agent, exhibiting concentration-dependent inhibition of multiple cancer cell lines with low toxicity to vascular smooth muscle cells. Structure-activity relationship (SAR) analysis revealed that para-halogenation (especially bromide) and methoxylation of phenylalanine enhanced cytotoxicity. This study provides valuable insights for the rational design of sansalvamide A analogs with improved pharmacological properties.