The causes of redness, swelling and pain in the mouth may be periodontitis, oral ulcers, mumps, etc., as follows. 1. Periodontitis: a chronic infectious disease of the periodontal tissues caused by plaque biofilm, when it develops to an advanced stage, deep periodontal pockets can be accompanied by periodontal abscess, at this time the periodontal tissues are swollen and the pain is more obvious. 2. Oral ulcers: the specific etiology is not clear, may be related to immunity, genetics, infection, environment and other factors, clinical features of the ulcer surface covered with yellow pseudomembrane, surrounded by a red halo band, the center of the depression, pain is obvious. 3. Parotitis: Staphylococcus aureus infection can lead to acute suppurative parotitis, which is characterized by enlargement of the parotid area and pressure pain. Parotid duct stones or foreign body blockage can lead to chronic obstructive parotitis, manifested by repeated swelling of the parotid gland, with slight pressure pain. There may be other causes of redness, swelling and pain in the mouth, and it is recommended to go to the hospital in time to clarify the cause of the disease and then carry out targeted treatment.