The invention provides novel medical application of 8-azaguanine (8-azaguanine, 8-AG), namely application of the 8-azaguanine to preparation of medicines for treating rheumatoid arthritis, and provides a medicinal composition containing the 8-azaguanine as an active component, wherein the 8-azaguanine having the structural formula is defined in the description; the weight proportion of the active component 8-azaguanine in the medicinal composition is 5-95%. Through measurement on in-vivo and in-vivo RA related factors, such as a rheumatoid factor (RF-IgM), an anti-cyclic citrulline (CCP) antibody, rheumatoid factor IgG and IgA, an anti-perinuclear factor, an anti-keratin antibody, an anti-nuclear antibody and an anti-ENA antibody, before and after medicine administration, results show that the 8-azaguanine has relatively high in-vitro and in-vivo rheumatoid arthritis treating activity. Therefore, the 8-azaguanine can be applied to the preparation of medicines for treating the rheumatoid arthritis, and has a relatively high clinical application value.