BACKGROUND Severe total colonic necrosis, septic shock and venous thromboembolism secondary to ulcerative colitis (UC) are rare and life-threatening. No such severe complications have been reported in the literature. METHODS We report a 36-year-old woman who developed total colonic necrosis and septic shock secondary to UC. The patient was treated with emergency surgery because computed tomography showed suspicious perforations. Persistent massive ascites occurred after operation and computed tomography angiography demonstrated portal vein, mesenteric vein and splenic vein thrombosis. The patient was discharged from hospital after active treatment. CONCLUSIONS Clinicians should pay attention to venous thrombosis, colonic necrosis and septic shock in UC patients. Close observation of surgical indications and timely surgical intervention are the key to reduce mortality and complications in UC.