The white substance above the oral ulcer is a pseudomembrane, mainly including fibrinous exudate and necrotic tissue, as follows. In the early stage of oral ulcer lesions, there are dense lymphocyte and monocyte infiltration in the mucosal epithelium and around the blood vessels, followed by polymorphonuclear leukocyte and plasma cell infiltration, and the epithelium is lysed and broken and dislodged, forming an ulcer. The ulcer stage of oral ulcer lesions, the fibrous exudate and necrotic tissue on the surface of the ulcer to form a pseudomembrane covering the lesion, which may also be mixed with inflammatory cells, bacteria, food residues, etc.. Pseudomembrane has a certain protective effect on the lesion area of oral ulcers, with the healing of oral ulcers, pseudomembrane can gradually disappear. If the symptoms of mouth ulcers are serious or prolonged, it is recommended to go to the hospital in time to receive early professional treatment.