When gastric juice reaches the mouth due to reflux for various reasons, the patient can feel acidity in the stomach, and the continuous feeling of acidity is called acid reflux for the following reasons: 1) relaxation of the lower esophageal sphincter, relaxation is mostly caused by increasing age; 2) esophageal lesions, under normal circumstances, if gastric juice refluxes into the esophagus, the lower end of the esophagus can remove gastric juice through peristalsis; 3) reduced saliva secretion, causing unfavorable gastric juice removal; 4) stomach problems, excessive gastric juice in the stomach or increased gastric pressure can appear as reflux.