To explore the mechanism of decoction (XWGD) decoction in treating rheumatoid arthritis (RA) in mice.Healthy male DBA/1 mice were used for CIA modeling. Twenty-five CIA mice with successful modeling and similar arthritis index (AI) scores were randomized equally into model group (CIA), methotrexate (MTX) group, and low-, medium-, and high-dose XWGD groups (0.975, 1.95, and 3.9 g/mL, respectively), with another 5 normal mice as the normal control group. The mice in normal control and CIA groups were given saline once a day, those in MTX group were given 0.1 mg/mL MTX once a week, and those in XWGD groups were treated daily via garage of XWGD containing crude drugs of different doses for 28 consecutive days. The AI score and HE staining were used to evaluate the changes in the joints of the CIA mice. The effect of XWGD on Th1, Th17, MDSC, G-MDSC and M-MDSC cells were evaluated with flow cytometry.Treatment with MTX and different doses of XWGD significantly decreased the AI score of the mice and relieved joint inflammation as compared with the model group ( < 0.05), and a higher dose of XWGD decoction produced a stronger therapeutic effect. Compared with those in CIA model group, the mice in MTX and XWGD treatment groups showed significantly decreased percentages of Th1, Th17 and M-MDSC cells in the spleen and increased percentages of G-MDSC cells ( < 0.01), and these changes were more conspicuous with a higher dose of XWGD. Correlation analysis showed that Th1 and Th17 cells were positively correlated with M-MDSC and negatively correlated with G-MDSC cells ( < 0.01).XWGD can improve joint inflammation in CIA mice by increasing the percentages of G-MDSC cells and decreasing the percentages of M-MDSC, Th1 and Th17 cells, and a high dose of XWGD can produce an equivalent therapeutic effect to methotrexate but with better safety.