Swelling and pain in the left eye may be caused by conjunctivitis, eye trauma, glaucoma, overuse of the eye, allergic reaction and so on. 1. Conjunctivitis: acute inflammatory reaction of the lid conjunctiva and bulbar conjunctiva caused by infection of the left eye with pathogenic microorganisms such as bacteria and viruses, and a large amount of inflammatory exudate leads to distension and pain of the left eye. 2. Ocular trauma: the left eye is affected by impact, extrusion and other traumatic injuries, resulting in damage to the eyeball, extraocular muscles, etc., and the tissue space is congested and edematous, so the left eye is distended and painful. 3. Glaucoma: the abnormal circulation of aqueous humor in the left eye leads to the increase of intraocular pressure, so it causes eye swelling and pain. 4. Excessive use of the eyes: excessive use of the eyes caused by physiological conditions such as staying up late for a long time, using electronic products such as cell phones, and frequent rubbing of the eyes may also cause swelling and pain in the left eye. 5. Allergic reaction: After the left eye comes into contact with pollen, hair, dander and other allergens, local allergic reaction is induced, resulting in edema, congestion, itching, swelling and pain in the left eye and other symptoms. Left eye swelling and pain may be related to a variety of factors, it is recommended to consult a doctor for a clear diagnosis.