The invention provides an application of lactococcus lactis in treating or preventing rheumatoid arthritis or related diseases thereof, and also provides lactococcus lactis containing pharmaceutical compositions, medicaments, foods and feeds. By providing lactococcus lactis for animals, an anti-inflammatory effect can be effectively played, the symptoms such as redness and swelling of joints are effectively improved, and the symptoms of arthritis are obviously relieved, and therefore, the lactococcus lactis can be effectively used for treating or preventing rheumatoid arthritis or related diseases thereof.