The causes of small blisters under the mouth include the following: 1, polymorphic exudative erythema: When suffering from polymorphic exudative erythema, erythema, papules and blisters can be formed on the inner side of the lips, which will soon break down and transform into a large area of erosion. 2, herpes zoster virus: When the herpes zoster virus attacks, dense herpes will be formed at the oral mucosa, and the ulcers will be larger and more painful after breaking down. 3, hand, foot and mouth disease: Hand, foot and mouth disease When the attack, the oral mucosa and other parts of the number of small blisters, 5-10 days to heal. 4, herpes stomatitis: with herpes stomatitis, can form clusters of dense small blisters in any part of the oral mucosa, the blister wall is thin and transparent, easy to break, the formation of superficial ulcers, and sometimes can develop into recurrent herpes stomatitis, and persist.