The most common cause of sudden blood in the stool is mixed hemorrhoids, especially internal hemorrhoids. It is possible to perform rectal diagnosis and, if necessary, anoscopy for a clear diagnosis. Second, the sudden appearance of blood in the stool may also be the cause of anal fissures, such as dry stool, easy to damage the skin of the anal canal and lead to anal fissures, so that when wiping stool with blood on the surface of the stool, blood is bright red, not mixed with stool. Third, the more common rectal polyps or rectal adenomas, long-term stool stimulation, but also easy to damage the polyp surface mucosa or submucosa blood vessels, resulting in blood in the stool. Fourth, rectal or colonic malignant tumors erode submucosal blood vessels, which may also lead to blood in the stool. Therefore, do not be nervous when blood in stool occurs, you should monitor the vital signs in time and give anoscopy or colonoscopy to clarify the cause if necessary.