This is a case implying a serious infectious complication risk during intensive severe ulcerative colitis treatment. A 26-year-old man developed diarrhea and bloody stool who was diagnosed with ulcerative colitis in 2018. He was managed with 5-aminosalicylic acid, but intolerance reaction resulted in discontinuation of treatment. He relapsed with severe abdominal pain and bloody stools in February 2019. He was referred to our department for intensive therapy. He had been treated with steroids, tacrolimus, granulocyte and monocyte apheresis, infliximab or tofacitinib, which temporarily improved his clinical symptoms. However, his medical condition could not be controlled. Hand-assisted laparoscopic subtotal colectomy was then performed in October 2019. He developed intermittent fever on postoperative day 3. Enhanced computed tomography (CT) revealed multiple deep vein thromboses and pulmonary embolism. Antibiotics and anticoagulation therapy were initiated, but postoperative day 13 CT showed multiple pulmonary cavities containing fluids and air, which were diagnosed as pulmonary abscess. His intermittent fever was over 38.0°C. Severe cough and hemoptysis lasted 3 weeks, the clinical symptoms and laboratory data then gradually improved after the fourth week.