The main causes of pain in the outer corner of one eye are blepharitis, canthal blepharitis and conjunctivitis. 1. Blepharitis: mostly caused by staphylococcal infection, it is an acute purulent lesion. It usually infects only one eye and is characterized by pain, redness and swelling at the outer corners of the eyes and a hard knot when gently touched. 2. Canthus blepharitis: mostly caused by bacterial infection or vitamin B2 deficiency. The disease can develop unilaterally or bilaterally, and its main clinical manifestations include itchy eyes, foreign body sensation, burning sensation, as well as the lid margin and skin of the outer corner of the eye are congested, swollen and painful. 3. Conjunctivitis: it can be caused by viruses, bacteria, mycoplasma and other infections or external stimuli. If the inflammation occurs in the conjunctiva at the edge of the outer corner of the eye, it can be manifested as conjunctival congestion, as well as pain in the outer corner of the eye accompanied by a small amount of mucus secretion. Therefore, it is recommended to go to a regular hospital to find out the cause and treat it promptly when there is pain in the outer corner of the eye.