To explore the mechanisms of astaxanthin alleviating the physiological abnormalities caused by blue body syndrome (BBS) in Litopenaeus vannamei, including body color abnormalities, growth retardation, and health deterioration, this study was conducted using two astaxanthin-supplemented diets: 0 mg/kg (control group, CT) and 100 mg/kg (astaxanthin group, AX). Juvenile shrimp [initial weight: (0.77 ± 0.27) g] were reared for 60 days in a recirculating aquaculture system. A systematic comparison of body color, astaxanthin content in various tissues, tissue structure, growth performance, antioxidant capacity, and immune function was performed between the two groups. The results showed: (1) Shrimp in the CT group exhibited blue body coloration, a typical symptom of BBS, whereas AX shrimp showed normal light brown coloration. During the trial, total astaxanthin content in the hepatopancreas, cuticle, and muscle tissues in CT shrimp decreased by 21.51%, whereas that in AX shrimp increased by 95.17%, with significantly higher astaxanthin levels in the hepatopancreas and cuticle (P＜0.05). At the end of the experiment, the blue-body ratio was 8.67% in the AX group, significantly lower than in the CT group (76.00%). These results suggest that exogenous astaxanthin supplementation can significantly increase astaxanthin deposition, improved body coloration, and reduced BBS incidence in shrimp. (2) Histological analysis revealed severe pathological changes in the CT group, including hepatopancreatic cell atrophy and deformation, loose muscle fiber arrangement, and detachment of the intestinal epithelial cells. In contrast, the AX group showed no tissue abnormalities, indicating that BBS caused structural damage to critical tissues, and astaxanthin supplementation helped maintain normal tissue structures. (3) Compared with CT shrimp, AX individuals showed significantly enhanced activities of digestive enzymes (amylase, lipase, and trypsin) and increased relative expression levels of growth-related genes (igf-1 and igf-2), resulting in improved growth performance (P＜0.05). These findings suggest that astaxanthin mitigates BBS-induced growth retardation by enhancing digestive enzyme activity and promoting the expression of growth-related genes. (4) AX shrimp exhibited significantly higher total antioxidant capacity, superoxide dismutase, and catalase activities, and lower malondialdehyde levels than CT shrimp (P＜0.05), indicating that astaxanthin ameliorated the oxidative imbalance caused by BBS through enhancing antioxidant enzyme activities. (5) Expression of immune-related genes, including crustin, penaeidin 3a, proPO, and relish, was significantly upregulated in AX shrimp compared with that in CT shrimp (P＜0.05), demonstrating that astaxanthin alleviates BBS-induced immunosuppression by promoting immune-related gene expression. In conclusion, nutritional metabolic disorders due to insufficient intake of astaxanthin are a critical cause of BBS in L. vannamei. Astaxanthin is preferentially utilized in shrimp to strengthen their antioxidant defense system, protecting tissue structure and physiological metabolism before being deposited for pigmentation, thus ameliorating the body color abnormalities and physiological dysfunctions associated with BBS.