High blood glucose and dry mouth are most likely caused by diabetes mellitus. Dry mouth should be actively hydrated, and glucose-lowering treatment is recommended at the same time. Typical manifestations of diabetes mellitus include thirst, excessive drinking, excessive eating, weight loss, etc. Thirst is mainly caused by elevated plasma osmolality as well as polyuria taking away a large amount of water, and when dry mouth occurs, it should be actively rehydrated, and glucose-lowering therapy should be performed at the same time. Patients should use hypoglycemic drugs, including metformin, acarbose, glimepiride, insulin, etc. under the guidance of doctors. Severe dry mouth in diabetes mellitus may appear hypertonic hyperglycemic syndrome, mainly characterized by severe hyperglycemia, high plasma osmolality, dehydration, and in severe cases, the patient may have psychoneurological symptoms, which requires timely medical attention. There are many other causes of dry mouth, such as dry syndrome, patients with high blood sugar with dry mouth should be diagnosed and treated under the guidance of a doctor.