Submucosal cysts in the left maxillary sinus are benign lesions that may form under the mucosa because of the inflammatory secretions that are always present inside the patient’s sinuses and irritate the mucosa of the sinus cavity. The cyst will gradually increase in size with the stimulation of inflammation and may even break down repeatedly. Sometimes patients will find yellow water secretion inside the nasal cavity after blowing their nose hard, but the patient’s symptoms may disappear after a few days, and after a period of time the symptoms may still recur. It is recommended that the patient should have surgery, which requires general anesthesia, to perform an opening of the inferior nasal tract to completely remove the cyst inside the maxillary sinus, and the recurrent symptoms of frequent yellow water should be significantly improved. It is recommended to avoid frequent rhinitis or sinusitis in life, which may lead to the formation of submucosal cysts in the maxillary sinus.