Left maxillary sinus cyst is a cystic, benign mass growing in the left maxillary sinus, including mucous cyst, mucosal cyst, and odontogenic cyst. 1. Mucous cyst: It is mainly caused by the long-term occlusion of the maxillary sinus opening, retention of secretions in the sinus cavity and poor drainage. It often causes eye pain, diplopia, facial deformity and other symptoms. Clinically, it is often necessary to remove the cyst through intranasal or extranasal approach and ensure the smoothness of sinus drainage. 2. Mucous membrane cyst: It is mainly due to the blockage of mucous glands or plasma glands under the mucous membrane of maxillary sinus, so that the material secreted by the glands accumulates and swells. Often appear sinus body surface bulging, overflow of tears, eye displacement and other manifestations. If the symptoms are not obvious, generally do not need surgical treatment; if the symptoms are more serious or recurrent, then the maxillary sinus radical pathway surgery. 3. Odontogenic cysts: they can be divided into dentigerous cysts and root cysts. Dentigerous cysts are mainly caused by abnormalities of tooth development, while root cysts are caused by degenerative changes of apical granulomas. Most of them have caries, dead teeth and other manifestations. Treatment of the former requires maxillary sinus cleaning, removal of the cyst and preservation of the mucosa of the maxillary sinus; the latter requires removal of the cyst and treatment of the diseased tooth. Once the left maxillary sinus cyst is found, it is recommended to consult the doctor as soon as possible to distinguish the type of disease, and then be actively treated.