A bag on the anus is often due to the following reasons: 1, perianal abscess: after suffering from perianal abscess, there will be a lump formation in the anus, at the same time there will be mild pressure pain, and even a throbbing pain, if you do a fine needle puncture can be extracted pus, you can go to the hospital’s general surgery or the anorectal surgery to make it clear. If the disease need to give antibiotics for treatment, while the surgical treatment, and then give regular medication changes. 2, hemorrhoids: can do anal fingerprinting, as well as anoscopy and other clear. Hemorrhoids can be treated conservatively when the condition is mild, and surgical treatment is needed when the condition is more serious.3. Swelling: there are swellings growing in the anus, such as benign swellings, or even malignant tumors, etc., which can be clarified by anus diagnosis, pelvic CT, etc., and surgical resection is usually needed to clarify the nature of the swelling.