Patients with one eye turning red may be due to subconjunctival hemorrhage, conjunctivitis, acute iridocyclitis and other causes. 1. Subconjunctival hemorrhage: patients with subconjunctival hemorrhage mainly show flaky redness of the white eyeball, usually without obvious discomfort, mainly caused by the rupture of small blood vessels in the conjunctiva after patients rub their eyes frequently or irritated by eye trauma. 2. Conjunctivitis: under the stimulation of inflammation, patients with conjunctivitis may show conjunctival congestion, which may also be accompanied by discomfort such as foreign body sensation and increased secretion in the eye. 3. Acute iridocyclitis: Patients with acute iridocyclitis may experience blurred vision, eye swelling and pain, photophobia and other discomforts, and may also experience redness of the eyes. Patients can follow the doctor’s instructions to use glucocorticoid eye drops and pupil-dilating eye drops for treatment, such as tobramycin dexamethasone eye drops, atropine ophthalmic gel and so on. Patients with severe monocular congestion are advised to seek prompt medical attention to evaluate their condition and follow medical advice.