Abstract: This study combined BFT with waterborne copper to explore the protective effects of different C/N ratios formed bioflocs of R. lagowskii on immunosuppression, inflammation and oxidative stress in acute copper exposure. The results showed that with the increase of C/N ratio, the activities of antioxidation, immune enzymes and anti-inflammation factor of each group increased at first and then decreased. Compared with the control group, the levels of serum alkaline phosphatase (AKP), acid phosphatase (ACP), lysozyme (LSZ), nitric oxide synthase (NOS), complement C3, C4 and immunoglobulin M (IgM) increased significantly in Ⅲ and Ⅳ groups (P<0.05). However, there was no significant difference in complement C3 and C4 content between Ⅲ and Ⅳ groups (P>0.05). The activities of serum catalase (CAT), ascorbic acid (ASA), superoxide dismutase (SOD), total antioxidant capacity (T-AOC), glutathione peroxidase (GSH-PX) and glutathione reductase (GR) enzyme in Ⅲ and Ⅳ groups were significantly higher than those in the control (P<0.05), while the content of malondialdehyde (MDA) was significantly lower than that in the control group (P<0.05). At the same time, the activities of SOD and CAT in treatment Ⅱ were significantly higher than those in the control group (P<0.05). The serum levels of tumor necrosis factor-α (TNF-α), interleukin-1 β (IL-1β) and interleukin-6 (IL-6) in Ⅲ and Ⅳ groups were significantly lower than those in the control group (P<0.05) without significant difference between Ⅲ group and Ⅳ group (P>0.05). However, the content of interleukin-2 (IL-2) in Ⅱ, Ⅲ and Ⅳ groups were significantly higher than that of the control group (P<0.05) with the highest content in group Ⅲ. Overall, these findings suggest that bioflocs can effectively enhance the activities of immune and antioxidant enzymes under acute copper exposure when C/N≥15 with the most significant effect at a 20﹕1 C/N ratio. Therefore, this study provides a new idea for in-depth exploration of bioflocs, a new mitigation scheme for water environmental pollution and biological toxicity caused by heavy metals, and a theoretical basis for promoting the healthy and stable development of aquaculture.