There are many reasons why half of the face is swollen and difficult to open the mouth, such as trauma, mumps, jaw arthritis, periodontal abscess and so on. 1. Trauma: If the patient has a history of trauma, it can cause half of the face swollen and difficult to open the mouth, and it may also be accompanied by localized bruising, bruising and other phenomena. 2. Mumps: Mumps is common in children and can cause half of the face to swell up and have difficulty opening the mouth, and there will be obvious difficulty in opening the mouth. 3. Mandibular arthritis: damage to the jaw joints can lead to half of the face being swollen with difficulty opening the mouth. 4. Periodontal abscess: Periodontal abscess can cause localized swelling, so the phenomenon of half of the face swollen with difficulty in opening the mouth will occur. There are many reasons why half of the face is swollen and difficult to open the mouth, and it is recommended to go to the hospital in time to improve the examination after the symptoms appear, and take standardized treatment under the guidance of a professional doctor, so as not to delay the condition.