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Background: Rheumatoid arthritis is an autoimmune disease that may lead to severe complications. The fruit of Psoralea corylifolia L. (PCL) is widely used in traditional Chinese medicine as a well-known herbal treatment for orthopedic diseases. However, there is a lack of studies of its effects on rheumatoid arthritis. The purpose of the study was to investigate the effects and mechanisms of concentrated herbal granules of PCL on rheumatoid arthritis to provide some insights for future development of new drug for the treatment of rheumatoid arthritis. Methods: We used collagen-induced arthritis (CIA) DBA/1J mice as an experimental model to mimic human rheumatoid arthritis. The mice were immunized with collagen on days 0 and 21 and then orally administered 200 mg/kg/day PCL on days 22–49. Starch was used as a control. The mice were sacrificed on day 50. Clinical phenotypes, joint histopathology, and immunological profiles were measured. Results: Compared to the CIA or CIA + Starch group, the CIA + PCL group had significantly ameliorated clinical severity and decreased paw swelling. Histopathological analysis of the hind paws showed that PCL mitigated the erosion of cartilage and the proliferation of synovial tissues. There were significant differences in the levels of TNF-α, IL-6 and IL-17A, as measured by ELISA, and the percentages of CD4 + IL-17A+, CD4 + TNF-α+, CD4 + IFN-γ+ T cells. Furthermore, we also found that in mice treated with CIA + PCL, the percentage and number of bone marrow-derived suppressor cells (MDSCs; Gr1+ CD11b+) increased significantly. Conclusions: We provided evidence for the potential antiarthritic effects of PCL through the inhibition of inflammation and increase of MDSCs. These findings indicate that PCL may be a promising therapeutic herb for the treatment of rheumatoid arthritis.