This study was conducted to evaluate the effects of different fibers, such as dietary pea, sweet potato, and wheat bran fibers, on the antioxidant capacity, immune status, and antioxidant-related signaling molecules of rat organs. In a 30 day experiment, a total of 44 rats were randomly divided into four dietary groups. These rats were fed either with a control diet or three basal diets supplemented with 15% pea fiber, 15% sweet potato fiber, and 15% wheat bran fiber. Results are as follows: (1) different fibers, such as dietary pea, sweet potato, and wheat bran fibers, can effectively promote antioxidant defense by decreasing free radicals and increasing non-enzymatic and enzymatic antioxidant capacities in relevant organs (P < 0.05). (2) Pea, sweet potato, and wheat bran fiber significantly increase the mRNA levels of enzymatic antioxidant substances in relevant organs. Moreover, supplementation of these fibers remarkably increased the NF-E2-related nuclear factor 2, Kelch-like ECH-associated protein 1, and mammalian target of rapamycin in the spleen (P < 0.05). (3) These fibers also significantly upregulated the anti-inflammatory mRNA level but downregulated the pro-inflammatory cytokine mRNA level in the jejunum, liver, and spleen. The mRNA level of transforming growth factor beta 1 also increased in rats with wheat bran fiber in different tissues (P < 0.05). In summary, pea fiber, sweet potato fiber, and wheat bran fiber supplements can improve the antioxidant capacity of plasma and relevant organs and regulate antioxidant-related signaling molecular expression in the jejunum, liver, and spleen of rats. Various fibers exerted different effects on immunity function in the jejunum, liver, and spleen by mediating the gene expression levels of anti-inflammatory and pro-inflammatory cytokines, TGF-β, and caspase 3.