Pain under the rib cage on the left side of the stomach is commonly caused by the following: a. Stomach malignant tumor. When gastric malignant tumor invades the whole layer of gastric wall, or retroperitoneal nerve, it will cause painful symptoms in patients, accompanied by nausea, vomiting and gastrointestinal bleeding. At this time, patients can clarify the nature of lesion through gastroscopy and perform radical treatment of gastric cancer plus regional lymph node dissection as early as possible. After the operation, combined with systemic chemotherapy, cancer cells in the body can be killed to reduce pain symptoms. Second, peptic ulcer perforation, mostly seen in gastric sinus perforation or duodenal bulb perforation. A large amount of gastric juice infiltrates into the abdominal cavity, which can cause acute diffuse peritonitis and cause painful symptoms under the left rib cage. Patients who are not treated promptly can die from infectious shock. Emergency surgery is required to repair the gastrointestinal perforation, flush the abdominal cavity cleanly, and intensify anti-infective treatment. 7-10 days, the patient’s perforated area can heal on its own.