Blastocystis hominis is an enteric parasite which has long been considered as an innocuous commensal living in the intestinal tract. Our research was to explore the role of B. hominis in refractory ulcerative colitis. Our department admitted 122 cases of ulcerative colitis patients. In these patients, there were 73 cases of patients who were responsive to sulfasalazinec, mesalazine in a standard dosage, according to the symptoms change. There was one patient who was detected to have B. hominis infection through stool detection. There were 49 patients with relapse symptoms. In this group, there were six patients who were detected with B. hominis infection through stool detection. The six patients of refractory ulcerative colitis were treated with metronidazole for 10-14 days. They almost completely recovered 3 weeks later. Patients diagnosed with ulcerative colitis should always consider this parasite infection when the symptoms are refractory and cannot be released.