Having black stool after bile duct stent is usually caused by upper gastrointestinal bleeding, taking medication, postoperative diet, and postoperative normal phenomenon. 1. Upper gastrointestinal bleeding: If patients have upper gastrointestinal bleeding after bile duct stenting, due to gastrointestinal bleeding the hemoglobin of trivalent iron ions in the blood turns into ferrous iron of divalent iron ions, and ferrous iron is mostly black in color, and the color of stools is black. 2. Taking medication: after biliary stenting, if the patient takes gastric medicine containing iron, it will lead to black stools. 3. Post-operative diet: After the surgery, if the patient consumes animal blood, animal liver, and a lot of vegetable (e.g. spinach) food, the color of the stool will become black. 4. Post-operative normal phenomenon: It is a normal condition after the surgery, and it is the black stool caused by the continuous flow of bile into the intestines. Black stool after bile duct stent may also be caused by bile duct blockage, bile excretion disorder, etc. Patients should go to the hospital in time under the guidance of the doctor to clarify the cause of standardized treatment.