Normal stool is generally cylindrical in nature, and the stool becomes flat for two years may be hemorrhoids, rectal polyps, chronic enteritis, rectal cancer and other causes. 1. Hemorrhoids: internal hemorrhoids lead to localized swelling of the anus can cause narrowing of the intestinal lumen, resulting in a flat stool shape. 2. Rectal polyp: it is a bulging lesion on the surface of rectal mucosa that protrudes into the intestinal lumen, and when the stool passes through the intestinal lumen containing rectal polyp, it may become flat. 3. Chronic enteritis: Chronic enteritis can also be accompanied by changes in the character of the stool, appearing unshaped stool, stool flattening and so on. 4. Rectal cancer: it is a kind of malignant tumor originated from rectal mucous membrane epithelium, there may be no obvious symptom in the early stage, but with the progress of the disease, there may be changes in the character of stools, blood in stools, emaciation, and so on, and it may also cause narrowing of intestinal lumen and lead to the flattening of stools. It is recommended to go to the hospital to clarify the cause of the flattening of the stool for more than two years to avoid delaying the condition.