Flat stool may be caused by internal hemorrhoids, rectal polyps, intestinal malignant tumors and other diseases. 1. Hemorrhoids: when suffering from hemorrhoids, especially internal hemorrhoids, in addition to perianal discomfort, blood in the stool, pain, perianal itching and other symptoms, in defecation, due to the stool by the anal vicinity of the congested and swollen venous plexus extrusion, coupled with the soft stool quality, can be the case of flat stool. 2. Rectal polyps: intestinal polyps are bulging organisms of the intestinal mucosa, when the diameter of the polyp is small, the symptoms of flat stool are not obvious. However, with the increasing size of the polyp, when the stool passes through the polyp area, it can be squeezed, thus the shape of the stool is flattened. 3. Malignant tumor of intestines: when there is a tumor in the intestines, the intestinal lumen will be narrowed with the gradual increase of the tumor, resulting in the feces being squeezed when it passes through the part of the tumor, which leads to flat feces. When the shape of stool is flat, it is recommended to go to the hospital in time for routine stool, colonoscopy and other related examinations, under the guidance of professional physicians. Under the guidance of professional physicians, the cause of the disease should be clarified and targeted treatment should be carried out to avoid delaying the condition.