The polypeptide DPP of Dendrobium aphyllum was obtained by microbial solid-state fermentation and sequenced by HPLC-MS/MS in our previous research, revealing six polypeptide sequences: DDDY, DYDD, MAK, AVTF, RSS, and MLCA. The relationship between the antioxidant function of DPP and its amino acid structure was investigated herein. To this end, the quantitative structure-activity relationship (QSAR) model was applied and an in vitro cellular antioxidant activity assay with HepG2 cells and erythrocyte hemolysis assay were performed to evaluate the antioxidant capacity of the peptide. Additionally, correlation analyses were performed to assess the effect of amino acid structure on the antioxidant activity. The results showed that all six synthetic peptides and DPP exhibited antioxidant activity (P<0.05). Among them, MLCA showed the strongest DPPH and ABTS radical scavenging ability, iron-reducing power, and erythrocyte hemolysis protection. The IC50 was 0.89, 0.99, 1.34, and 0.68 mg/mL, respectively. Meanwhile, the AVTF peptide exhibited the strongest cellular antioxidant activity with an EC50 of 0.02 mg/mL. Hydrogen- and electron-donating amino acids and hydrophobic amino acids contributed to the antioxidant activity of DPP. The favorable amino acids at each point included: Met as the N-terminal amino acid; Leu or Val as the second amino acid at the N-terminal; Leu as the third amino acid at the C-terminus. In addition, Ala contributed to the antioxidant activity of the peptides, whereas changes in the Tyr position had minimal impact on the antioxidant activity. This study establishes a theoretical framework for comprehensive investigation of the structure-activity relationship between antioxidant peptides, laying the foundation for further research on antioxidant peptides.