Oral ulcer is an ulcerative disease of the oral mucosa characterized by periodic recurrent episodes, and its history is characterized by recurrent, periodic and self-limiting nature. It has clinical features such as yellow, red, concave and painful. Since there is no basis for specific and substantial laboratory tests as well as pathological examinations for oral ulcers, laboratory tests and biopsies are generally not required. However, most scholars advocate routine blood tests to enable timely detection of oral ulcers caused by malnutrition or blood disorders, gastrointestinal diseases, etc. For large and deep oral ulcers with long duration, the possibility of cancerous ulcers should be alerted, and biopsy is needed to clarify the diagnosis if necessary.