A submucosal cyst of the left maxillary sinus may be a maxillary sinus mucus cyst or a maxillary sinus retention cyst. 1. Mucous cyst of maxillary sinus is a benign cyst-like lesion, which is formed when the ducts of mucous glands in the mucosa of the maxillary sinus are obstructed, resulting in the slow aggregation of mucous secretion, which is usually large in size and spherical in shape, and it has the potential of destroying the bony tissues and damaging the neighboring tissues with expansive growth, and it is rare in the maxillary sinus. 2. Retention cyst of maxillary sinus is a benign plasma cyst, which is thought to be caused by the obstruction of the ductal opening of the plasma glands in the mucosa of maxillary sinus, small in size, hidden in the sinus opening, not easy to be detected by imaging, and it does not invade the surrounding tissues. The left submucosal cyst of the maxillary sinus is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment or treatment under the guidance of the doctor.