Anal pain and diarrhea may be caused by anal fissure, proctitis or acute gastroenteritis. 1. Anal fissure: Frequent constipation may cause the patient’s anal opening skin tissue to crack, thus causing anal fissure, resulting in pain on the inside of the anus, accompanied by diarrhea as well as blood dripping from the anus and other uncomfortable symptoms. 2. Proctitis: unclean eating habits may induce inflammation of the rectal area by the infection, thus causing the patient to have diarrhea, blood in the stool and the left side of the abdominal pain, diarrhea more often will also cause pain in the inner anus. 3. Acute gastroenteritis: often overeating or drinking a lot of alcohol, may induce acute gastroenteritis, the seriousness of the condition will cause the patient to appear repeatedly diarrhea, accompanied by anal swelling, thus causing pain in the inner anus. Inner anal pain diarrhea, it is recommended to go to the hospital to do the relevant examination, to clarify the cause of the disease, under the guidance of the physician for treatment, to avoid aggravation of the condition.