There are more causes of acidity in the mouth. Oral mucositis, gingivitis, calculus, dental caries and other diseases stimulate the secretion of oral glands, and the secreted fluid stays in the oral cavity for too long, and then the acidity in the mouth is felt. With chronic gastritis or duodenal ulcer, the secretion of gastric acid increases, and if it is accompanied by esophageal fissure insufficiency, excessive gastric acid will flow back into the esophagus, leading to reflux esophagitis. Due to the stimulating effect of gastric acid, the esophageal mucosa instantly produces a large amount of fluid to neutralize gastric acid, and the esophagus is in spasm under the stimulation of gastric acid. The refluxed gastric acid mixed with the fluid secreted by the esophagus will partly enter the stomach and partly go upward into the mouth, and the mouth will have an acidic sensation. To sum up, it is recommended to keep cleaning the mouth every morning and evening, and also go to the hospital for examination to find out the specific cause of acidity in the mouth and treat it from the root.