The cause of black stool in advanced liver cancer is gastrointestinal bleeding, and the black stool is caused by the blackening of the upper gastrointestinal tract after the mixing of feces and gastric juice. And the relative amount of bleeding is not very large, it is a continuous, chronic bleeding caused by black stool. If the amount of bleeding is large, it appears as vomiting blood, so the cause of black stool is chronic bleeding in the upper gastrointestinal tract. The causes of chronic bleeding in the upper gastrointestinal tract are chronic bleeding caused by tumor invasion of the stomach wall leading to tumor surface rupture, and also bleeding caused by stress ulcers due to long-term poor feeding of liver cancer patients. It is also possible that most of the liver cancer is caused by cirrhosis, and most of the cirrhotic patients will have varices in the esophagogastric fundus due to portal hypertension, which may cause bleeding and black stool if it ruptures.