Stomach cancer patients excreting black stool can be changed after surgery. If the tumor cells of gastric cancer invade blood vessels in the stomach and cause bleeding in the upper gastrointestinal tract, and under the action of gastric acid, a chemical reaction occurs and turns into black color, resulting in the patient excreting black stools. To deal with this situation, patients should actively cooperate with attending doctors to treat the primary lesion of gastric cancer, such as radical surgical resection treatment, which can remove the whole stomach or a part of the stomach, and the black stool will disappear after the surgery. After gastric cancer resection surgery, patients should also combine with doctor’s comprehensive treatment plan to carry out follow-up chemotherapy, radiotherapy or targeted therapy, etc., to inhibit further spreading and metastasis of cancer cells and effectively control the development of the disease. Once stomach cancer patients have black stools, it means their condition has become more serious, so it is recommended to go to regular hospitals as soon as possible. Cooperate with doctors to carry out systematic treatment so as to avoid delaying the condition.