A blister in the anus is considered to be one of the following possibilities. First, it may be herpes simplex, a disease caused by infection to the herpes simplex virus at the anus. It is easy to appear especially when the patient has low immunity, and it is necessary to use anti-viral ointment to apply to the affected area. Secondly, it may be caused by skin allergy, especially in allergic patients due to local skin metaplasia, which leads to the dilation of skin capillaries and the formation of blisters due to the penetration of plasma under the skin. Thirdly, it may be a perianal abscess, which will appear as a darker purulent blister and needs to be treated in time, or else the abscess will expand in size or ulcerate and flow. Fourth, there may be bacterial infections and other triggers, it is best to go to the hospital to do a careful examination to distinguish, and then carry out the corresponding treatment is good.