Objective To investigate the effect of inhibitor of differentiation 2 (Id2) on the proportion of CD4+T cells by detecting the proportion of CD4+T cell subsets and Id2 expression in peripheral blood and joint synovial fluid of patients with rheumatoid arthritis (RA). Methods A total of 51 RA patients (including 18 patients providing synovial fluid) and 31 healthy controls (HCs) were enrolled. The proportions of CD4+T cells, Th1 cells, and Th17 cells, and their expression of Id2 in peripheral blood and synovial fluid of RA patients and HCs were detected by flow cytometry. Results Compared with HCs group, the proportions of circulating CD4+T cells, Th1 cells, and Th17 cells and their expression of Id2 in RA patients did not change significantly. The proportions of CD4+T cells and Th1 cells, and Id2 expression in CD4+T cells in synovial fluid of RA patients were significantly higher than those in peripheral blood of RA patients and HCs. The expression rate of Id2 in CD4+T cells was positively correlated with the expression of IFN-γ, but not with erythrocyte sedimentation rate (ESR), C reactive protein (CRP), and Disease Activity Score 28 (DAS28). Conclusion CD4+T cells are enriched in RA synovial fluid, and their Id2 expression may promote Th1 cell differentiation.