Bacteroides fragilis (BF) is a symbiotic bacterium in human intestinal tract, and also an anaerobic bacterium with relatively high positive rate in clinical work. 11 strains of anaerobic bacteria isolated from 8 cancer patients from June 2020 to December 2021 were identified as Bacteroides fragilis by matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS ) technique. Analysis results of medical records showed that: five patients suffered with colorectal cancer; Surgery is the most common predisposing factor of Bacteroides fragilis bloodstream infetion (4/8); Imipenem and cilastatin sodium was selected for treatment in 4 patients, and the other 4 patients took other different types of antibiotics. Seven patients who effectively treated with antibiotics got controll of fever and infection. Their reexamination results of blood culture turned to be negative. This case analysis of bloodstream infection caused by Bacteroides fragilis in cancer patients can provide evidence for microbiology detection and clinical diagnosis and treatment.