The causes of blisters in the mouth are herpetic stomatitis, herpetic pharyngitis, hand-foot-mouth disease, eating hard, overheated food, and aspergillosis, as follows: 1. Herpetic stomatitis: small clusters of small blisters, such as the size of a pinhead, are widely formed in the oral mucosa, which can easily break down and appear as superficial ulcers. 2. Herpetic pharyngitis: clusters of small blisters can form on the soft palate and suspensory, which can heal in about 7 days. 3, Hand-foot-mouth disease: A large number of small blisters will appear on the oral mucosa, which will heal in 5-10 days. 4, hard, overheated food stimulation: chewing hard food can rub the lip mucosa and produce blisters. Sometimes eating food with too high a temperature can also burn the mucosa and form blisters. 5, aspergillosis: when the body’s immunity decreases and aspergillosis or aspergillosis-like sores occur, the oral mucosa will form a large volume of blisters, and the wall of the blisters will easily break down and tear off together with the surrounding normal tissue, exposing a bright red wound.