Objective: To investigate the serum lactate level in patients with rheumatoid arthritis (RA) and its relationship with disease activity, and to analyze the effect of sodium lactate on the activation of CD4⁺ T cells, the ability of secreting cytokines and CD4⁺T cell subsets in peripheral blood of the RA patients. Methods: The peripheral blood of healthy controls (HC) and RA patients was collected, and the content of lactate in the supernatant was detected by lactate detection kit, the correlation between the content of lactate and the disease score of the RA patients was analyzed; the activation level of CD4⁺ T cells, the proportion of CD4⁺ T cell subsets and the cytokines secreted by CD4⁺ T cells in peripheral blood of all the RA patients were detected by flow cytometry after being stimulated with sodium lactate. Results: The serum lactate level in the RA patients (n=66) was significantly higher than that in the HC (n=60, P < 0.001), and there was a certain correlation with disease activity score in 28 joints (DAS28)-C-reactive protein (CRP) (r=0.273, P=0.029), The levels of rheumatoid factor [RF, 197.50 (26.03, 783.00) IU/mL vs. 29.30 (0.00, 102.60) IU/mL, P < 0.01], CRP [37.40 (11.30, 72.60) mg/L vs. 5.83 (2.36, 12.45) mg/L, P < 0.001], were increased in patients with the lactate concentration greater than 5 mmol/L were significantly higher than those in patients with the lactate concentration less than or equal 5 mmol/L, however, there was no significant difference in the expression of erythrocyte sedimentation rate [ESR, 42.00 (19.00, 77.00) mm/h vs. 25.00 (12.50, 45.50) mm/h, P>0.05] and anti-cyclic citrullinated peptied (CCP) antibody [82.35 (17.70, 137.00) RU/mL vs. 68.60 (25.95, 119.70) RU/mL, P>0.05]. Compared with the control group, the expression of PD-1 (46.15%±8.54% vs. 41.67%±9.98%, P < 0.001), inducible costimulatory molecule (ICOS, 5.77%±8.60% vs. 18.65%±7.94%, P < 0.01) and CD25 (25.89%±5.80% vs. 22.25%±4.59%, P < 0.01) on the surface of CD4⁺ T cells in the RA patients treated with sodium lactate was significantly increased. Compared with the control group, the proportion of Th17 (4.62%±1.74% vs. 2.93%±1.92%, P < 0.05) and Tph (28.02%±6.28% vs. 20.32%±5.82%, P < 0.01) cells in CD4⁺T cells of the RA patients in the sodium lactate treatment group increased. Compared with the control group, the expression of IL-21 (5.73%±1.59% vs. 4.75%±1.71%, P < 0.05) in CD4⁺T cells was up-regulated in the RA patients treated with sodium lactate. Conclusion: The level of serum lactate in RA patients is increased, which promotes the activation of CD4⁺T cells and the secretion of IL-21, and up-regulates the proportion of Th17 and Tph cells in the RA patients.