Objective This study was designed to explore the therapeutic effect of psoralen on type II collagen-induced rheumatoid arthritis in mice and its molecular mechanism. Methods DBA/1J mice were immunized with type II bovine collagen to induce rheumatoid arthritis. The model mice were randomly divided into Psoralen group (PSO), methotrexate group (MTX) and model group (Vehicle). Clinical signs of arthritis in the mice were monitored. The spleen index was assessed. Splenic Th1 and Th2 cells were counted by flow cytometry. ELISA was used to detect the levels of inflammation-associated factors TNF-α, IL-6 and IL-1β in the serum. Results Compared with the vehicle group, the ankle swelling and limitation of joint activity in the PSO group were significantly reduced, the spleen index and Th1 cell percentage were significantly decreased, and the Th2 cell percentage showed no significant change in the PSO group. Expression of TNF-α, IL-6 and IL-1β in serum was notably decreased in the PSO group. All the indexes showed no significant difference between the PSO and MTX groups. Conclusions Psoralen may attenuate the severity of type II collagen-induced rheumatoid arthritis in mice by regulating the balance of Th1/Th2 cells and inhibiting the expression of TNF-α, IL-6 and IL-1β.