Swelling of half eyelid may be caused by allergic reaction, mosquito bite, blepharitis, dacryoadenitis and so on. 1. Allergic reaction: As one side of the eye comes into contact with allergic substances, such as pollen, willow, etc., it may lead to eyelid allergy and symptoms such as eyelid swelling, itching and pain. 2. Mosquito bites: As the eyes are bitten by mosquitoes, the venom may be injected into the eyelids, so it leads to eyelid swelling, and most of them are accompanied by itching symptoms. 3. Blepharitis: due to not paying attention to eye hygiene, rubbing the eyes with dirty hands, etc., staphylococcus bacteria may lead to infection of the eye, acute suppurative inflammation occurs, which can be manifested as redness, swelling, heat and pain of the eyelids. 4. Dacryoadenitis: Inflammation of the lacrimal gland due to viral or bacterial infection, which can be categorized into acute dacryoadenitis and chronic dacryoadenitis, both of which can lead to swelling of the eyelids. Acute lacrimal gland may also show symptoms such as pain, tearing discomfort. If the symptoms of eyelid swelling can’t be relieved or are accompanied by other symptoms, consult a doctor in a timely manner, clarify the cause of the disease and follow the doctor’s instructions to standardize the treatment, so as to avoid delaying the condition.