There are many reasons for the swelling and pain of one side of the nostril, and the common ones are nasal vestibulitis, nasal boils, and foreign objects in the nasal cavity. 1. Nasal vestibulitis: diffuse inflammation of the skin of the nasal vestibule, which can be unilateral or bilateral, manifested as redness, swelling and pain of the skin at the nasal vestibule, accompanied by skin erosion or cracks in severe cases. 2. Nasal boils: mostly caused by nose picking, pulling out nasal hair or trauma to the mucous membrane of the nasal vestibule or the external nasal skin appendage injury, the most common in the nasal vestibule, manifested as local redness, swelling, heat, pain, limited elevation, severe pain, sometimes accompanied by low-grade fever and general malaise. 3. Nasal foreign body: Nasal polyps, cysts, tumors and other foreign bodies will appear on the lesion side of the nasal cavity swelling and pain symptoms. One side of the nostril swelling and pain may also exist for other reasons, it is recommended to go to the ear, nose and throat in a timely manner to the doctor, improve the examination to clarify the cause of the disease, to give targeted treatment or treatment.