Lower gastrointestinal bleeding is clinically subdivided into six major categories, which are inflammatory, mechanical, vascular, neoplastic, and systemic. Most of the lower gastrointestinal bleeding comes from the colon and rectum, and a few from the small intestine. The characteristics of the common diseases are briefly described as follows: (1) Colon and rectal polyps or colonic polyposis: intestinal polyps mostly occur in the rectum and sigmoid colon, most of them are single and a few are multiple. The main symptom is blood in the stool. Blood in stool is characterized by intermittent, bright red color, generally small amount, not mixed with feces. Children with blood in the stool, but the number and nature of the stool is basically normal, most of them are rectal polyps. Colon polyps are mostly seen in young adults, whose clinical features are diarrhea, fresh blood and mucus in stool, and can cause anemia due to repeated bleeding. (2) Colonic hemangioma: The main symptom is intestinal bleeding or intestinal obstruction. It may manifest as acute hemorrhage, and anemia due to long-term small amount of blood loss is common. The hemangioma can be capillary hemangioma or cavernous hemangioma, or both. Colonoscopy reveals large extended veins under the mucosa. (3) Amoebic colitis: The patient’s main symptom is hemorrhage, the stool is saucy red, with a lot of mucus and a foul odor, and there is often pressure pain in the right lower abdomen, and the urgency is less than that of bacillary dysentery. (4) Rectal and colon cancer: rectal cancer blood in stool is bright red or dark red in early stage, not much, intermittent, often with alternating history of constipation and diarrhea; in late stage, the stool is often mixed with foul-smelling mucus at the same time. Left-sided colon cancer is similar to rectal cancer, but is prone to bowel obstruction. Right-sided colon cancer is mainly characterized by tarry stools, and the stool occult blood test is often positive, accompanied by indigestion, anemia and right lower abdominal masses and other signs. (5) Colonic diverticulitis: mostly located in the sigmoid colon, with habitual constipation. Uncomplicated diverticula are asymptomatic, but if the diverticula are inflamed, there may be mucus and blood stools, abdominal pain, fever and other symptoms. (6) Ulcerative colitis or other inflammatory diseases: large amount of blood in the stool is rare, often mucus blood stool, with a history of diarrhea, abdominal pain, mostly accompanied by lower abdominal pain, colonoscopy and barium enema angiography can be clearly diagnosed. (7) Internal hemorrhoids: The first and second stage internal hemorrhoids are mainly blood in the stool, blood in the stool usually occurs during defecation, sometimes dripping, shooting blood, or only blood on the hand paper, blood and feces do not mix, bright red color. Stage III internal hemorrhoids are often accompanied by symptoms of hemorrhoid prolapse. The hemorrhoids bleed in small amounts, usually without urgency and pain, and often with constipation symptoms. (8) Intestinal trap: often mucus and blood stool, jam-like, common in children, accompanied by abdominal pain, abdominal masses can be found in the trap, easy to cause intestinal obstruction. (9) Meniere’s diverticulum: also known as terminal ileal diverticulum, is a congenital disease, characterized by sudden blood in the stool, without abdominal pain or other symptoms, can be relieved by supportive therapy, but can soon bleed again. Bloody stools are often dark red with blood clots and cause shock and anemia when there is a lot of bleeding. This disease should be thought of for unexplained lower gastrointestinal bleeding in pediatric patients. (10) Small intestinal tumor: benign tumor is rare, usually no systemic symptoms, and less bleeding. If the tumor is enlarged, it can cause black or red blood in the stool, often accompanied by abdominal distension, abdominal pain, loss of appetite, abdominal mass and other symptoms. (11) Anal fissure: bleeding during defecation, accompanied by anal pain, small amount of bleeding, bright red color, history of constipation, more common in young people, local examination can be seen in the anal canal fissure, mostly in the posterior median or anterior median. (12) Radiation proctitis: blood in the stool, urgency, mixed blood and mucus, frequent defecation, and experience of radiotherapy.