Submucosal cysts in the left maxillary sinus are due to long-term secretion of inflammatory substances in the maxillary sinus, which stimulates inflammatory proliferation of the mucosa in the sinus cavity and the formation of cysts under the mucosa, which are benign lesions with common causative factors such as chronic rhinitis, sinusitis or chronic periapical infection of the maxillary molar. As bacteria can cause thickening of the mucosa, a large amount of mucus is formed, inducing the mucosa to produce fibrous tissue differentiation and eventually the formation of larger cysts. If small cysts do not cause significant symptoms, cephalosporin and nitroimidazole antibiotics can be applied to control the bacterial infection and root canal treatment of the focal tooth and, if necessary, extraction of the tooth with a poor prognosis. If the cyst increases in size causing blockage of the sinus cavity and causing symptoms such as headache and dizziness, the gum can be incised along the maxillary vestibular groove so as to separate the mucosa, remove the cyst and then suture the wound closed.