The Anguilla japonica is a migratory fish species of significant economic importance. In intensive aquaculture systems, unfavorable rearing conditions can lead to immunosuppression, rendering the species more susceptible to pathogenic infections. Moreover, following stock enhancement releases, the transition from artificial aquaculture environments to natural ecosystems exposes A. japonica to challenges such as food scarcity and predation, which may compromise its survival and adaptability. Proper flow training during artificial rearing and prior to stock enhancement has been demonstrated to improve health status and enhance survival capacity in the wild to some extent. To date, no studies have examined the effects of flow training on the antioxidative capacity and immune system of A. japonica. Additionally, research on the behavioral biology of adult A. japonica remains limited. This study investigates the relationship between critical swimming speed and body length, as well as the effects of different flow velocities on oxygen consumption rate, energy consumption per unit distance, antioxidative capacity, nonspecific immunity, and energy metabolism. Using swimming performance testing equipment and circular flow systems, physiological parameters were measured under four flow velocity conditions: static water (0.0 m/s), 0.1 m/s, 0.2 m/s, and 0.3 m/s. The results revealed a linear positive correlation between critical swimming speed (U_crit) and body length, with the relationship modeled as: U_crit=1.44L+12.57. The relative critical swimming speed (U_crit') was described by the equation: U_crit'=–0.004 1L+1.896 0. Additionally, the relationship between water velocity and oxygen consumption rate (Mo₂) was expressed as: Mo₂=0.005 8V²+3.808 3V+34.709 0, while the cost of transport (COT) was modeled as: COT=1.145 0×10^–5V² – 0.001 6V + 0.185 8. Regarding antioxidant indices, increases in water velocity significantly enhanced total antioxidant capacity (T-AOC), hydroxyl radical scavenging capacity (HFR-SC), reduced glutathione (GSH) levels, and catalase (CAT) activity in the serum of A. japonica. Notably, T-AOC and CAT activities in the 0.3 m/s group were significantly higher than in the still water group, while GSH levels peaked at 0.2 m/s. In terms of liver enzyme activity, glutamate dehydrogenase (GDH) activity increased with velocity, peaking at 0.2 m/s before declining. Meanwhile, pyruvate kinase (PK) activity was significantly higher in the 0.3 m/s group compared to the still water group (P<0.05). These findings suggest that appropriate flow training (0.2–0.3 m/s) enhances the antioxidative capacity and energy metabolism efficiency of A. japonica, promoting healthier growth and improving survival in natural environments. This study provides valuable guidance for health management during A. japonica aquaculture and flow training design prior to stock enhancement.