Subconjunctival hemorrhage, conjunctivitis, keratitis, sclerositis, uveitis, acute attacks of angle-closure glaucoma can all be monocular in onset, and trauma to one eye can also cause congestion in one eye. Conjunctival congestion is a very common sign in ophthalmology. Inflammation, trauma, and various ophthalmic diseases may manifest as conjunctival congestion, especially diseases of the anterior segment. Among the various diseases with conjunctival congestion, several common monocular diseases are listed: subconjunctival hemorrhage is mainly manifested as a patchy red bruise on the white eye, usually without the shape of blood vessels, diffuse, not affecting vision, and without significant ocular discomfort, mainly due to the rupture of small blood vessels under the conjunctiva; conjunctivitis, keratitis, sclerositis, uveitis if one eye is infected with germs can be manifested as monocular Congestion: if the eye does not affect the vision of the eye red eye grinding with discharge is considered conjunctivitis, if accompanied by blurred vision eye irritation symptoms are considered keratitis, uveitis, such as local congestion and pressure pain is considered scleritis, such as inflammation caused by congestion; In addition, acute angle-closure glaucoma usually first attack in one eye, then the performance of one eye is obvious congestion, accompanied by vision loss, eye swelling pain, headache, nausea and vomiting, etc., because the eye is not affected. In addition, fatigue, alcohol, constipation, excessive eye use, hot water baths, etc., can cause the expansion of blood vessels, which often manifests as double eye congestion, but can also be monocular, which is physiological rather than pathological. In summary, single eye congestion is not specific, and the causes of its occurrence are basically the same as simultaneous congestion in both eyes, but some diseases are mostly monocular in onset, which can be used as a basis for differential diagnosis based on the accompanying other manifestations.