A cyclic analog of natural peptide Yunnanin A was synthesized via photoinduced single electron transfer reaction (SET) in the paper. The resulting compound exhibited potent bioactivity (with IC₅₀ values 29.25 μg mL⁻¹ against HepG-2 cell lines and 65.01 μg mL⁻¹ against HeLa cell lines), but almost have no toxicity to normal cells (with IC₅₀ values 203.25 μg mL⁻¹ against L929 cell lines), which may be served as a potential antitumor drug for medical treatment. The spatial structure was examined by experimental electronic circular dichroism (ECD) and quantum chemistry calculations. Moreover, the theoretical study suggested that special intramolecular hydrogen bonds and γ, β-turn secondary structures may be possible sources affecting cyclic peptide's bioactivity.