Rectal cancer can cause blood in the stool, but there are more reasons for blood in the stool, and the time of bleeding in the stool does not have much correlation with rectal cancer. There are more causes of blood in stool, such as digestive tract diseases, including massive bleeding from upper gastrointestinal tract, inflammation of intestines as well as hemorrhoids and anal fissures, etc. In addition, some blood system diseases can also cause blood in stool, such as thrombocytopenia, etc., but none of these diseases will develop into rectal cancer. When patients have intestinal polyps and intestinal ulcers, if they are not actively treated and bleed for a long time, they may develop cancer. Rectal cancer mainly manifests as change in bowel habit, thin stool, bloody stool, pus-blood stool, urgency, constipation, diarrhea, etc. The diagnosis requires colonoscopy and pathological biopsy, and patients should not rely on the time of stool bleeding to make a diagnosis. Patients are advised to go to the hospital promptly when there is bleeding in stool to clarify the cause of blood in stool and to provide active treatment to prevent deterioration of the condition.