Sudden swelling of the upper eyelid of one eye may be caused by mosquito bites, acute injuries, allergies and other factors. 1. Mosquito bites: in the summer, mosquitoes are more, the upper eyelid by mosquito bites, there will be a sudden swelling of the symptoms, may also be accompanied by redness, itching. 2. Acute injury: external blow or bump injury, resulting in damage to the upper eyelid, the upper eyelid will be suddenly swollen, accompanied by pain, subcutaneous bruising and other symptoms, and may even lead to eye bleeding. 3. Allergy: contact with allergic substances, resulting in eye allergies, will make the upper eyelid redness and swelling, accompanied by itching, improper care is also prone to secondary bacterial infection. The upper eyelid of one eye is suddenly swollen, it is recommended that timely medical examination and treatment, so as not to cause other diseases.