The thinning and darkening of stools may be related to congenital megacolon and rectal cancer. 1. Congenital megacolon: this disease is mainly the structure of environment and genetic joint action, mainly due to the abnormal development of fetal embryo, abnormal microenvironment of intestinal wall, etc. Patients often have persistent constipation, which can be manifested as thin and dark stool. Patients often suffer from persistent constipation, which can be manifested as thinning and darkening of stools, and some of them are accompanied by abdominal distension and other symptoms. 2. Rectal cancer: the cause of this disease is still unclear, and it is mostly considered to be related to diet, age, heredity, chemical carcinogens, digestive diseases, lifestyle and other factors. Early stage patients usually have no obvious symptoms, and with the development of the disease, frequent bowel movement, change of bowel habit, urgency and heaviness, and feeling of anus dropping before defecation can also occur. If there is a narrowing of the intestinal lumen, it will be manifested as progressive thinning of the stool and darkening of the color. It is recommended that when the phenomenon of thinning and darkening of stools occurs, patients should consult a doctor in a timely manner to clarify the cause of the disease under the guidance of a professional physician and follow the doctor’s instructions for treatment.