The invention relates to the technical field of an application of bacteroides fragilis (BF) 839, in particular to an application of the bacteroides fragilis 839 to preparation of a medicine or food for treating or adjunctively treating immune-related diseases. The key effect of the bacteroides fragilis 839 in human intestinal tracts in treatment or adjuvant treatment of the immune-related diseasesis discovered for the first time, and the bacteroides fragilis 839 can be made into forms of pharmaceutical compositions, foods, health-care products or food additives by utilizing the key effect ofthe bacteroides fragilis 839 in the immune-related diseases. A pharmaceutical composition, a food, a health-care product or a food additive contains the bacteroides fragilis 839, can be used for treating the immune-related diseases, and has very important application value.