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In this study, protein from Porphyra haitanensis was used as raw material to prepare an antioxidant peptide, and its anti- oxidant activity was evaluated in vitro. A model of H2O2-induced oxidative damage in HepG2 cells was established, and the effects of Porphyra haitanensis hydrolysates (PHHs) on superoxide dismutase (SOD) activity and malondialdehyde (MDA) content were de-tected. Finally, the structure of PHHs was identified by ESI-MS/MS. The results showed that the 1,1-diphenyl-2-pyridylhydrazine (DPPH)-free radical-scavenging ability of PHHs was the strongest (59.28％ at 1.0 mg mL-1) when hydrolyzed with an acidic protease for 4 h. PHHs with different concentrations had protective effects on H2O2-induced damage to HepG2 cells, and the protective effect was enhanced with increasing concentrations. When the level was 400 μg mL-1, the cell survival rate was as high as 88.62％. More- over, PHHs can significantly reduce oxidative damage to HepG2 cells by H2O2, improve SOD activity, and reduce MDA content. The tetrapeptide Asp-Lys-Ser-Thr, with a molecular weight of 448 Da, was identified as an important fraction of PHHs by high-resolution mass spectrometry.