Abstract: Stocking density is a key factor affecting the growth, health, and rearing environment of farmed fish. As a functional feed additive, fructooligosaccharides (FOS) can effectively alleviate rearing stress, promote growth, and enhance antioxidant capacity. To investigate the effects of different stocking densities and FOS supplementation on the growth performance, antioxidant capacity, and intestinal microbiota of mandarin fish (Siniperca chuatsi), an 8-week indoor recirculating aquaculture experiment was conducted. Mandarin fish with an initial body weight of (29.19±0.47) g were allocated into five groups: low density (LD, 1 kg/m³), medium density (MD, 3 kg/m³), high density (HD, 5 kg/m³), high density with 1% FOS (HF1, 5 kg/m³+1% FOS), and high density with 2% FOS (HF2, 5 kg/m³+2% FOS). The results showed that the survival rate (SR), weight gain rate (WGR), specific growth rate (SGR), and feed efficiency (FE) of the LD and HD groups were significantly lower than those of the MD group (P<0.05), indicating that both low and high densities inhibited growth performance and feed utilization. Compared with the MD group, the activities of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-PX) in the liver of the LD and HD groups were significantly increased (P<0.05), suggesting that the fish maintained homeostasis by enhancing antioxidant capacity. Regarding the intestinal microbiota, Fusobacteriota was the dominant phylum in all density groups, but the abundances of Proteobacteria, Firmicutes, and Actinobacteriota in the LD and HD groups were significantly increased, indicating a trend toward diversification of the microbial community. Compared with the HD group, both HF1 and HF2 groups showed increases in WGR, SGR, FE, and condition factor (CF), with significant differences observed in the HF2 group (P<0.05), indicating that 2% FOS supplementation significantly improved growth performance, feed utilization, and condition factor. Moreover, the HF2 group exhibited significantly increased hepatic SOD and CAT activities, demonstrating that FOS supplementation enhanced antioxidant capacity. In addition, the 2% FOS group showed significantly increased alpha diversity of the intestinal microbiota, and PCoA revealed a distinct separation of the microbial community structure, with a significantly increased proportion of beneficial bacteria (e.g., Bacillus), indicating a significant shift in the microbial structure (P<0.05). In conclusion, both high and low stocking densities significantly reduced growth performance and feed utilization of mandarin fish, and the fish maintained homeostasis under density stress by enhancing antioxidant capacity. Dietary supplementation with 2% FOS effectively alleviated high-density stress, improving growth performance, feed utilization, antioxidant capacity, and intestinal microbiota diversity. These findings provide a theoretical basis for the healthy aquaculture and nutritional regulation of mandarin fish under high-density conditions.