Objective:To evaluate the effect of Yiyiren Decoction on the prevention and treatment of collagen induced arthritis with syndrome of cold-dampness obstruction (CIA-CDO) its mechanism.Methods:A cold dampness obstruction type CIA rat model was prepared by injecting type Ⅱ collagen and simulating a cold and humid environment in an artificial climate chamber. The successfully modeled rats were randomly divided into a model group, a positive group, and Yiyiren Decoction high- and low-dosage groups according to a random number table method. Additionally, 8 normal rats were set as the blank group. The Yiyiren Decoction high- and low-dosage groups were orally administered with Yiyiren Decoction at dosages of 24 and 12 g/kg, the positive group was orally administered with leflunomide at a dosage of 10 mg/kg, and the blank group and model group were orally administered with equal volumes of normal saline once a day for 4 consecutive weeks. The overall signs of the rats were observed daily, and the arthritis index (AI) of the limbs and paws was assessed weekly; after 4 weeks of drug administration, the serum levels of pro-inflammatory cytokines (TNF-α and IL-1β) and anti-inflammatory cytokines (IL-4 and IL-10) were detected by ELISA, histopathology of the ankle joint sections was observed under the light microscope, and gene expressions of synovial MMP-3 and TIMP-1 were determined by RT-qPCR.Results:The overall physical signs and joint symptoms of rats in the Yiyiren Decoction high- and low-dosage group increased; compared with the model group, after 3 and 4 weeks of administration, the AI scores of the Yiyiren Decoction high- and low-dosage group and positive group decreased ( P<0.01); the Yiyiren Decoction high- and low-dosage group showed varying degrees of improvement in inflammatory cell infiltration, synovial/fibrous tissue proliferation, and bone and cartilage structure damage in the ankle joint; compared with the model group, the levels of pro-inflammatory cytokines TNF-α and IL-1β in the high and Yiyiren Decoction low-dosage groups decreased ( P<0.01), while the levels of IL-4 and IL-10 increased ( P<0.01); the mRNA level of MMP-3 in synovium decreased ( P<0.05 or P<0.01), while the mRNA level of TIMP-1 increased ( P<0.05 or P<0.01). Conclusion:Yiyiren Decoction can improve the joint symptoms of CIA rats with cold-dampness obstruction, and its preventive and therapeutic effects may be related to correcting the imbalance of pro-inflammatory/anti-inflammatory cytokines and MMP-3/TIMP-1, reducing the inflammatory reaction in the joint cavity and the whole body, and inhibiting the metabolic disorder of joint matrix.