Abstract: Mytilus coruscus is a shellfish with huge economic importance in China, and the mantle is an important immune-related organ. To investigate the immune function of the mantle and its underlying mechanisms, peptidoglycan (PGN) was used as a stressor, and ultra-high-pressure liquid chromatography-mass spectrometry technique was employed to identify the composition and content of metabolites from the mantle at 48h of post peptidoglycan stress. In addition, free amino acids, antimicrobial activities, and antioxidant activities of the mantle were analyzed and compared between the control and stressed samples. The results revealed that PGN stress induced significant changes in mantle metabolites, and a total of 486 metabolites with significant difference (SDMs) was identified, including 232 up-regulated and 254 down-regulated SDMs (P<0.05). Among these, lipids and lipid−like molecules, organic acids and their derivatives, and organo-heterocyclic compounds were the most prominent SDMs. KEGG enrichment analysis further revealed that these SDMs were enriched in different pathways, showing a complex response of the mantle to the PGN stress. Notably, the up-regulated SDMs were enriched in pathways related to cell signaling and amino acid metabolism, while down-regulated SDMs were enriched in lipid metabolisms, vitamin related metabolisms, and autophagy. Gene set enrichment analysis (GSEA) further confirmed the regulation of some SDMs and the enrichment of aminoacyl-tRNA biosynthesis and autophagy-animal. In addition, free amino acid analysis also confirmed the up-regulation (P<0.05) of certain amino acids, such as aspartic acid, glutamic acid, and arginine, in the mantle under PGN stress. Enzymatic activity analysis revealed an increase (P<0.05) of catalase activity and a decrease of hydrogen peroxide in the PGN stressed mussel mantle. Interestingly, antimicrobial function analysis revealed that the mucus from PGN stressed mantle presented stronger inhibition against Bacillus subtilis, indicating that PGN stress induce stronger antibacterial activity in mussel mantle. These findings indicated that the mussel mantle may respond to PGN stress by enhancing immune capacity, regulating inflammatory balance, and inhibiting cell autophagy. This study provides valuable insights into the molecular strategies of mussel mantle in response to immune stress and offers a scientific basis for future efforts to promote healthy aquaculture of mussels.