The most common cause of vomiting bitter water is gastroesophageal reflux disease, which means that stomach acid in the stomach lumen and duodenal fluid containing bile and pancreatic enzymes flow back into the esophagus and are then vomited out by the mouth, which can manifest as vomiting bitter water. The two most common causes of GERD are the following: First, abnormal anti-reflux defense mechanisms in the normal esophagus, the most important of which is the lower esophageal sphincter. Under normal conditions, the lower esophageal sphincter maintains a very high pressure and reduces the frequency of gastric acid reflux into the esophageal lumen, but under specific circumstances, structural abnormalities or inability of the lower esophageal sphincter to maintain high pressure in its normal function can lead to reflux of gastric acid and bile into the esophagus. Second, poor gastric motility leads to the absence of normal emptying of gastric acid and duodenal fluid in the gastric lumen, and a relatively large amount increases the chance of reflux into the esophagus, leading to the occurrence of gastroesophageal reflux disease.