Constipation and blood on the surface of stool are considered to be related to hemorrhoids, anal fissure, rectal polyps, rectal cancer and other causes. 1. Hemorrhoids: constipation will lead to a rapid increase in abdominal pressure during defecation, which can easily cause hemorrhoids to rupture and bleed, especially when the stool is too dry, it is easy to cut through the surface of the hemorrhoidal nucleus, causing bleeding. 2. anal fissure: constipation is more difficult to discharge stools, and stools are often dry, when discharged, it is easy to lead to perianal skin tearing, thus causing bleeding. 3. Rectal polyp: the surface of rectal polyp itself is covered by mucous membrane, if the stool is dry when you are constipated, it will easily cause the mucous membrane of rectal polyp to rupture and bleed. 4. Rectal cancer: when the rectal cancer lesion develops to a certain degree, it may cause small blood vessel damage, resulting in bleeding in stool, especially when constipation occurs, it is easy to cause bleeding due to friction of small blood vessels in stool. Constipation and blood on the surface of stool may also be caused by other reasons. It is recommended to go to the hospital in time to find out the reasons under the guidance of professional doctors and carry out targeted treatment.