Mouth ulcers are a common disease in the oral mucosa, prone to recurrence and related to autoimmunity and nutritional deficiencies. Recurrent oral ulcers are a kind of recurrent mouth sores, with localized redness, swelling and pain in the oral mucosa, which can affect eating and normal life in severe cases. There are many causes, the common one is caused by low immunity, when the body resistance is lowered such as the cold, it will induce mouth ulcers. Mouth ulcers also have certain genetic factors, one of the parents often mouth ulcers, the children have a greater chance of developing mouth ulcers. Mouth ulcers are also related to vitamin deficiencies in the body, such as vitamin C deficiency, which can also lead to mouth ulcers, or micronutrient deficiencies such as iron deficiency, which can lead to increased brittleness of mucous membranes and can easily trigger mouth ulcers. Mouth ulcers are also related to mental stress. Mental anxiety or emotional stress, excessive stress in life, or negative emotions can trigger mouth ulcers. In addition, bad restorations or residual crowns, sharp tips, etc. can easily trigger traumatic mouth ulcers. It is necessary to remove the irritating factors as soon as possible to avoid the cancer of the oral mucosa caused by the long-term bad stimulation. Generally speaking, oral ulcers can heal on their own in about two weeks, which may be related to immunity, heredity, mental stress, etc. If the ulcer does not heal in more than two weeks, it should be seen by an oral and maxillofacial surgery specialist to exclude local irritation factors, and if necessary, biopsy should be taken to alert the possibility of cancer.