Ulcerative colitis (UC) is a non-specific chronic colitis, which is a common and frequent disease. The Chinese Society of Gastroenterology has repeatedly standardized the diagnostic criteria of UC. 2000 Chengdu meeting further revised the diagnostic criteria of UC (referred to as “criteria”), and was published. However, the clinical work does not follow the “standard”, the phenomenon of arbitrary diagnosis still occurs from time to time, there are often misdiagnosis. Common errors in diagnosis are as follows: 1. Infectious colitis is misdiagnosed as UC. Infectious colitis is an inflammation of the colon with a clear cause of infection, such as bacterial, viral or parasitic infection caused by colitis, commonly bacterial dysentery, amoebic colitis and tuberculosis of the large intestine. If these infectious colitis of clear etiology is mistakenly diagnosed as non-specific UC and treated with immunosuppressants or even corticosteroids, the mistakes can be imagined. In actual clinical cases, some endoscopic manifestations of infectious colitis can resemble UC, which can be misdiagnosed if not carefully differentiated. Therefore, the criteria emphasize the need to exclude some infectious diseases with known causes for the diagnosis of this disease. UC and infectious colitis are distinguished by: (1) the duration of the disease and clinical manifestations are different: the duration of UC more than 6 weeks, for persistent or recurrent episodes; infectious colitis often has an epidemiological history, the duration of the disease generally does not exceed 4 weeks, after antibiotic treatment rarely recur; (2) the search for pathogens: the search for lysoamebic trophozoites in fresh stool; in 50% of patients with infectious colitis stool (3) Differentiation from colonic mucosal biopsies: UC has abnormal crypt structure and irregular arrangement of glandular epithelium, which is rare in acute intestinal infections in general. In the differential diagnosis of UC and tuberculosis, the distribution of the lesions is from the distal rectum to the proximal colon, and the lesions are continuous with shallow ulcers. In the case of intestinal tuberculosis, the lesions are segmental, usually in the ileocecal region, and tend to be disseminated, isolated and deep ulcers. The pathological changes of tuberculosis are also different from those of UC, with epithelioid granulomas and caseous necrosis, and sometimes acid-resistant bacteria can be found. Combined these characteristics of the two is not difficult to distinguish. 2, the Crohn’s disease (CD) or Behcet’s disease and other immune-related diseases misdiagnosed as UC. To make the distinction requires familiarity with the diagnostic criteria of CD and Behcet’s disease. In general, neither is a continuous lesion. the pathological features of CD and UC are completely different, the former being inflammation of the entire intestinal wall with lacunar ulcers, lymphocytic aggregates in the submucosa, vascular and lymphovascular hyperplasia, and non-caseating necrotizing granulomas. In contrast, UC is a more superficial lesion of the mucosa and submucosa, with pathological changes different from CD: in addition to inflammatory cells, neutrophils and eosinophils infiltrating the lamina propria, UC may have cryptitis, formation of crypt abscesses with glandular epithelial hyperplasia, and cupulocytopenia. Behcet’s disease is mostly disseminated, isolated ulcers with more prominent intestinal mucosal vasculitis changes. The two can mostly be differentiated by clinical and pathological diagnosis. The above-mentioned conditions are sometimes not good even if they are not distinguishable from UC, because the two treatment principles are basically the same, both take immunosuppressive therapy. 3, some inflammation of the colon misdiagnosed as UC. Some patients with some inexplicable inflammation of the colon, such as minor local congestion of the mucosa, erosion, was crowned as “colitis”, misdiagnosed as UC, which is a mistake should not happen. If you open any medical textbook, there is no simple diagnosis of “colitis”, only the aforementioned specific or non-specific colitis. We should avoid such errors. In case the lesion does not meet the diagnostic criteria and there are doubts, close follow-up observation should be done according to the “criteria”, or some general antibiotics should be used first for short-term follow-up observation. Once the diagnosis is made, long-term treatment with hormones or immunosuppressive drugs should be used. Therefore, the adverse effects of long-term medication and the economic burden of the patient should be considered and weighed before treatment, and inappropriate diagnosis and excessive treatment will bring greater losses to the patient and should be avoided.