The causes of vomiting in rectal cancer patients are as follows: 1. With the increase of tumor and tumor volume, the tumor gradually grows into the lumen, then the narrowing of intestinal lumen can occur, and obstruction will occur in serious cases. When this situation occurs, patients will stop anal defecation, defecation, abdominal pain, and then intestinal contents can not pass through the rectum, and patients’ abdominal distension will also be aggravated, and vomiting will occur; 2. Some patients do not have intestinal obstruction, but due to the change of defecation habit of patients with rectal cancer, and their poor nutritional status, electrolyte disorders, especially hypokalemia, can also occur. The common clinical symptom caused by hypokalemia is abdominal distension, which can also cause vomiting; 3. If patients have vomiting, they will also lose potassium, and patients with potassium loss will aggravate abdominal distension, which will then aggravate vomiting. If the patient has vomiting, we should pay attention to identify what causes the vomiting, we should do a standing abdominal radiograph to see if there is intestinal obstruction. Check electrolytes to see if there are electrolyte disorders, and then according to the results of the examination for targeted treatment. If the vomiting is caused by intestinal obstruction, surgery is usually recommended. If it is caused by electrolyte disorders, the electrolyte disorders should be corrected in time to prevent more serious complications.