Astaxanthin is an important aquatic feed additive that enhances the antioxidant capacity, and immune function of rainbow trout (); however, very limited information is available on its underlying molecular mechanisms. powder, powder, and synthetic astaxanthin were added to the commercial feed (no astaxanthin, NA) to prepare three experimental feeds, referred to as the HPA, PRA, and SA groups, respectively, and their actual astaxanthin contents were 31.25, 32.96, and 31.50 mg.kg, respectively. A 16-week feeding trial was conducted on the with an initial body weight of 669.88 ± 36.22 g. Serum and head kidney samples from commercial-sized were collected for metabolomics and transcriptomics analysis, respectively. Metabolomics analysis of the serum revealed a total of 85 differential metabolites between the astaxanthin-supplemented group and the control group. These metabolites were involved in more than 30 metabolic pathways, such as glycerophospholipid metabolism, fatty acid biosynthesis, linoleic acid metabolism, and arginine and proline metabolism. It is speculated that different sources of dietary astaxanthin may regulate antioxidant capacity and immunity mainly by affecting lipid metabolism and amino acid metabolism. Transcriptomic analysis of the head kidney revealed that the differentially expressed genes between the astaxanthin-supplemented group and the control group, such as integrin beta-1 (), alpha-2-macroglobulin (), diamine acetyltransferase 1 (), CCAAT/enhancer-binding protein beta () and DNA damage-inducible protein 45 alpha (), which are involved in cell adhesion molecules, the signaling pathway, phagosomes, and arginine and proline metabolism and play regulatory roles in different stages of the antioxidant and immune response of .