If you find diarrhea with black thin stool, consider whether it is caused by gastrointestinal bleeding, you need to go to the hospital for timely consultation, the possible causes are as follows: first, gastric ulcer or duodenal ulcer, because ulcers cause damage to the mucosa of the gastrointestinal tract, there is vascular damage, which can cause gastrointestinal bleeding, blood in the gastrointestinal tract after mixing with intestinal fluid and gastric juice, the stool will be black, bleeding is relatively large, the stool will be thin, the color can appear dark red; second, in addition to the upper gastrointestinal tract, small intestinal lesions or colon lesions bleeding, can also cause blood in the stool, and the appearance of thin, black stool.