A swollen eye can be caused by insect bites, or by diseases such as granuloma and contact dermatitis. 1. Insect bites: When one side of the eye is injected with secretions by insect bites such as mosquitoes, bedbugs, mites, fleas, etc., then swelling will occur. Itching, erythema and other symptoms. 2. Granuloma: this disease is mainly caused by Staphylococcus aureus infection, common triggering factors are hands uncleaned rubbing the eyes, eye make-up removal is not timely, decreased immunity. Granuloma usually refers to the occurrence of one side of the eye, the local will appear red, swelling, heat, pain and other symptoms, can be manifested as a swollen eye. 3. Contact dermatitis: This disease is an allergic reaction caused by skin contact with exogenous substances. If one side of the patient’s eye comes into contact with allergens, such as pollen, dust, eye creams, etc., redness, swelling, itching, burning and other symptoms are likely to occur. In addition, one eye is swollen also need to consider the trauma, glomerulonephritis, blepharitis and other causes. Patients with swollen eyes are advised to visit a hospital to determine the cause of the swelling and to seek appropriate treatment.