Vague pain on the left side of the abdomen is seen in the following cases: First, ulcerative colitis occurs more often in young people, and it is an autoimmune disease. The vague pain in the abdomen is accompanied by diarrhea, blood in the stool, and even mucopurulent stool. The diagnosis can be clarified by colonoscopy, which reveals local ulceration, congestion and even erosion of the intestinal mucosa. Systematic treatment should be carried out, oral immunosuppressants, regular colonoscopy, as well as conventional treatment and life and diet modification. Secondly, partly caused by tumors of the intestine, especially malignant tumors, will manifest as hidden pain, loss of appetite, weight loss, and even localized masses will be found during physical examination. And patients with advanced stage are prone to distant organ metastasis or extensive abdominal metastasis and obvious abdominal distension.