One flat and one bulging eye is considered a unilateral protrusion of the eyeball. The most common causes of unilateral protrusion of the eyeball are mainly two: 1. It is an inflammatory protrusion of the eyeball, which is commonly caused by orbital cellulitis and can also be caused by inflammatory pseudotumor. Inflammatory protrusion of the eyeball requires only anti-inflammatory treatment and does not require surgery. 2. Tumor protrusion of the eyeball mainly includes hemangioma, which is the most common benign tumor in the orbit, and cavernous hemangioma is the most common. In addition, there are also lacrimal gland tumors, dermatomal cysts, meningiomas, optic nerve gliomas, and orbital sarcomas. Tumorous proptosis of the eye requires surgery in most cases. Finally, it should be noted that in case of monocular protrusion, it is recommended to go to a regular ophthalmology institution for further treatment in a timely manner.