The causes of pain in one side of the mouth are stomatitis, temporomandibular arthritis, bite muscle spasm and so on. 1. Keratitis: Keratitis is characterized by redness, wetness and even erosion of the corners of the mouth, which, when serious, can lead to pain and difficulty in opening the mouth, etc. Unilateral keratitis can cause pain in one side of the mouth. 2. Temporomandibular arthritis: unilateral temporomandibular arthritis will have pain in the temporomandibular joint, popping when opening the mouth, impaired jaw movement, and so on, and in severe cases, the mouth cannot be opened. 3. Bite muscle spasm: unilateral bite muscle spasm, there will be the mouth can not open, one side of the pain, and there will be oral and maxillofacial twitching, bite muscle pain and other discomforts. There are many reasons why the mouth can not open one side of the pain, patients should be timely consultation, early treatment, so as not to delay the diagnosis and treatment.