With more than 20 years of clinical experience, I would like to summarize the occurrence of diarrhea, abdominal pain and belching after major gastrectomy for gastric or duodenal ulcer. First of all, most of these patients are diagnosed as “gastrojejunocolonic fistula”, and the diagnosis depends on gastrointestinal imaging, with the highest rate of barium enema examination, up to 90%. Fiberoptic gastroscopy or colonoscopy is also a good diagnostic tool. Secondly, most of these patients present with epigastric pain, belching, diarrhea, anemia, lethargy, and even cachexia, with diarrhea being one of the most characteristic clinical manifestations, with an increase in the number of diarrhea, mostly 5-8 times/day, and in some cases up to 10 times, with dilute watery stools, mostly with incompletely digested food, which can occur between 20 min and 2 h after eating. A fecal odor is often present in the belching. The main reason is the recurrence of ulcers in the gastrojejunostomy anastomosis, which then gradually invades the colon and eventually forms an endovascular fistula, so the reason why some patients are prone to recurrence is that the extent of resection at the time of major gastrectomy was not enough to effectively reduce the amount of gastric acid secretion. Furthermore, the distal jejunum is less tolerant to gastric acid and more prone to ulceration, and a very small number of patients have a combination of gastrinoma, which should be excluded preoperatively. Finally, the only effective treatment for this disease is surgery.