The corrosion inhibition performance and mechanisms of five cyclic amino acids, namely tryptophan, histidine, proline, phenylalanine, and tyrosine, for Fe in HCl solution were investigated by means of molecular dynamics (MD) simulations and density functional theory (DFT) calculations. The results demonstrate that all the five cyclic amino acids exhibit significant corrosion inhibition effect. Among others, tryptophan (Trp) exhibits the most outstanding inhibition performance, which may be attributed to the unique π-electron conjugation effect of its indole ring and the strong coordinating ability of its amino group. MD simulations reveal that Trp forms a dense adsorption film on the metal surface, with a significantly higher adsorption energy (-381.45 kJ/mol) rather than other amino acids (e.g., histidine: -307.11 kJ/mol). Its molecular orientation facilitates the construction of a hydrophobic barrier, effectively impeding the ingress of H⁺/Cl^- ions. DFT analysis further elucidates that the highest occupied molecular orbital (HOMO) of Trp is localized on the indole ring and amino group, enabling strong chemical bonding with vacant d-orbitals of metal Fe via electron back-donation. Conversely, the lowest unoccupied molecular orbital (LUMO) is distributed over the indole ring and carboxyl group, enhancing its adsorption stability. Trp's global reactivity parameter (ΔN = 0.456) is considerably higher than that of other amino acids (e.g., proline ΔN = 0.325), indicating superior charge transfer capability. This results in a pronounced synergistic effect suppressing both anodic metal dissolution and cathodic hydrogen evolution. Consistent with the theoretical findings, electrochemical evaluation confirms that Trp also delivers the best corrosion inhibition performance.