When one side of the anal opening is swollen, it may be due to perianal abscess, thrombosed external hemorrhoids and other diseases. 1. Perianal abscess: usually due to acute purulent lesions caused by pathogenic bacterial infection, during the onset of the disease, the patient may be swollen on one side of the anal opening phenomenon. 2. thrombosed external hemorrhoids: the cause of this disease is not clear, may be due to infection, improper diet and other factors, in the acute attack period, may cause the patient to appear in the anal opening side of the phenomenon of swelling. When the anus is swollen on one side, there may be other causes, it is recommended that the patient consult a doctor in a timely manner, to clarify the cause and diagnosis, under the guidance of the physician to give reasonable standardized treatment.