〔Abstract〕 The purpose of the experiment was to analyze the changes of interleukin-21(IL-21) in peripheral blood samples and joint microenvironment of patients with osteoarthritis(OA) and healthy controls by ELISA, and to detect the percentage of CD3+CD8-IL-21+T lymphocytes in peripheral blood mononuclear cells and infiltrating CD3+CD8-IL-21+T lymphocytes in synovium of OA patients and healthy controls by flow cytometry. Compared with healthy volunteers, the concentration of IL-21 in serum and joint microenvironment of OA patients increased, and the percentage of CD3+CD8-IL-21+T cells in peripheral blood mononuclear cells in total lymphocytes increased. At the same time, the percentage of CD3+CD8-IL-21+T cells in synovial infiltrating lymphocytes in OA patients also increased. It was found that the expression of IL-21 increased in patients with OA, and might be involved in the pathogenesis of the disease by increasing abnormal immune cascade reaction.