Chronic colitis is a recurrent, chronic episode or persistent inflammation of the colonic mucosa, and many different causes can lead to chronic inflammation of the colonic mucosa. The etiology of chronic colitis may be due to infectious factors, immune disorders, allergies, ischemic factors, etc. The symptoms are mainly abdominal pain and diarrhea, and the stool pattern varies depending on the cause, ranging from normal stools to yellow stools, bloody stools, and mucus stools. The degree of abdominal pain varies. For patients with diarrhea, abdominal pain, and changes in stool shape, colonoscopy should be considered first. Because colonoscopy and tissue biopsy are the main tools for diagnosing the disease. The severity and characteristics of mucosal inflammation vary depending on the etiology. In ulcerative colitis, the inflammation ends in the rectum, and the mucosal inflammation involving the intestinal segments is diffuse. In contrast, the inflammation in Crohn’s disease is segmental; the mucosa between the two lesions is approximately normal. Ischemic colitis mostly involves the descending colon or sigmoid colon, while the rectum is rarely involved. The inflammation in chronic colonic evolution can be congested and edematous, and in severe cases, there may be erosions and ulcers, or even strictures. In contrast, mild chronic colitis may be apparently abnormal in the afternoon when observed by the naked eye, and inflammatory cell infiltration can only be seen microscopically through biopsy pathology. The diagnosis of the etiology of chronic colon is very important, and the cause of the disease can be clearly identified in order to give targeted treatment.