Swelling of the lower side of the left eyelid may be caused by physiological edema, blepharitis, soft tissue contusion, allergy and other reasons. 1. Physiological edema: due to poor sleep or sleeping with a pillow too low, the blood return to the face is affected, resulting in edema under the left eyelid. 2. Blepharitis: Bacterial infection may occur due to inattention to eye hygiene or decreased resistance of the patient or refractive error. Patients may have redness, swelling and pain under the eyelids. 3. Soft tissue contusion: Due to impact or abrasion on the left eyelid area, it may lead to redness of the skin of the left eyelid and obvious hematoma under the eyelid. 4. Allergy: If you come into contact with allergens, such as pollen, dust mites, cosmetics, etc., it may cause eye allergy, so there are symptoms such as redness, swelling and itching under the left eyelid. When the left eyelid is swollen underneath and has uncomfortable symptoms, you should go to the hospital in time and ask the doctor to check and treat it clearly.