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This experiment was conducted to study the effects of Astragalus polysaccharide (APS) on intestinal antioxidant capacity, immunity, structural integrity, and permeability of hybrid crucian carp (control group, Astragalus polysaccharide stress group (APs), and salt and alkali stress group (SAs)) under saline-alkali environment. The results showed that the CAT activity, MDA activity, and total antioxidant capacity in the APs group was significantly higher than that in the control group (P < 0.05). At 6 h, the activity of intestinal GPX in the SAs group was significantly higher than that in the control group and APs group (P < 0.05). At 48h, the activity of C3 in the APs group was significantly higher than that in the control group (P < 0.05). At 24 h, the content of C4 in the APs group was significantly higher than that in the control group (P < 0.05). At 72 h, the ACP and the AKP activity in the APs group and the SAs group was significantly higher than that in the control group (P < 0.05). Inflammatory cells infiltrated the folds in both the APs and SAs groups. At 72 h, the gene expression levels of claudin 7, claudin 12, claudin c, and ZO-1 in the APs group and SAs group were significantly higher than those in the control group (P < 0.05). At 72 h, the expression of inflammation-related genes in the APs group and SAs group was significantly higher than that in the control group (P < 0.05). In conclusion, APS, as an additive, can alleviate the damage of intestinal physical barrier and immune barrier caused by salt-alkali stress to hybrid crucian carp, including up-regulation of genes for inhibiting inflammation and tight junction and alleviating oxidative stress, thus maintaining the integrity of intestinal structure. [ABSTRACT FROM AUTHOR]