Immunity due to immune balance contributes to disease prevention and treatment. Radix Codonopsis polysaccharide (RCP) is isolated from the root of the Chinese herb Codonopsis pilosula. Previous studies have indicated that RCP has immunomodulatory activities; however, the effects of RCP on immunity, especially immune balance, are still largely unknown. In the present study, we investigated the effects of RCP on T-cell balance in mice. The mice were pretreated intragastrically with or without RCP for 15 days and injected with hydrocortisone on days 10-15 to disturb the immune system. The spleen and thymus were weighed and used to calculate immune organ indexes. The percentages of CD4(+) T cells, CD8(+) T cells, Th1 cells, Th2 cells, regulatory T cells (Tregs) and Th17 cells in peripheral blood were assayed by flow cytometry. Pro-inflammatory and anti-inflammatory cytokines, TNF-α, IL-1β and IL-10, in serum were determined by enzyme-linked immunosorbent assay (ELISA) kits. The results showed that RCP pretreatment could maintain the homeostasis of CD8(+) T cells, Tregs, Th17 cells, TNF-α, IL-1β and IL-10 in hydrocortisone-treated mice. Furthermore, RCP pretreatment maintained the immune balance of CD4(+)/CD8(+) T cells, Th1/Th2 cells, Tregs/Th17 cells, IL-10/TNF-α and IL-10/IL-1β. Taken together, RCP pretreatment had beneficial effects on the maintenance of T-cell balance against hydrocortisone disturbance in mice and potential to be developed into novel functional food.