The cause of yellow stool first and then black may be caused by blood in the stool, common diseases are: 1, duodenal bulb ulcer, the symptoms of the disease is mainly epigastric discomfort, chronic onset, often with night pain, pain improves after eating, etc., when the ulcer erodes the surrounding blood vessels can produce varying degrees of bleeding symptoms, mild manifestations of black stool; 2, colon cancer, bloody stool is often the earliest symptoms of the disease, or dysentery-like The most common symptoms of this disease are intermittent blood in the stool or with shortness of breath, followed by mucus when secondary infection occurs, and polyps that are too large can lead to intestinal obstruction and intestinal overlap; 4, small intestinal ulcer, the most common symptoms of this disease are abdominal distension, belching, abdominal pain around the umbilicus, blood in the stool, etc. It is recommended to improve endoscopy; 5, diet, whether recently Whether there has been eaten animal offal, can be considered as a cause.