Bleeding from the external opening of an anal fistula is considered to be a possible cause of blood, pus or purulent discharge due to inflammatory irritation of the external opening during an acute episode of anal fistula. It is mainly related to the persistent infection of the inner opening as well as acute suppuration. If patients with anal fistula do not undergo surgical treatment, it is possible to have localized external and internal persistent infection during the acute attack. When the internal opening becomes infected, inflammatory secretions and some blood in the anal canal pass through the fistula tract, which leads to outflow of infection along the fistula tract to the external opening, and a flow of secretions and blood from the external opening occurs. The secretions may be purulent, bloody or pus and blood etc. not necessarily. Even in some patients, during the acute suppurative infection stage, there is a possibility of redness, swelling, heat and pain in the skin around the external opening, which affects normal life. When anal fistula patients have external bleeding or pus and local skin infectious symptoms, it is recommended to carry out perianal ultrasound examination as soon as possible to clarify the branches and direction of the fistula as soon as possible to carry out anal fistula surgery, to avoid the spread of infection aggravation.