Rheumatoid arthritis is an autoimmune disease in which methotrexate, an important immunosuppressant, plays a very effective role in the treatment. The principle of methotrexate in the treatment of rheumatoid arthritis is that, as an inhibitor of folate reductase, it mainly inhibits dihydrofolate reductase, while preventing the reduction of dihydrofolate to biologically active tetrahydrofolate, thus blocking the transfer of one-carbon groups in the biosynthesis of purine and pyrimidine nucleotides, resulting in the inhibition of DNA biosynthesis. It also has an inhibitory effect on thymidine nucleotide synthase, but a weaker effect on the inhibition of RNA and protein synthesis. The treatment of rheumatoid arthritis mainly involves the inhibition of T-lymphocyte activity, thereby suppressing abnormal immune activity.