There are several reasons for constant thirst, which may be related to abnormal salivary gland secretion, or the manifestation of systemic diseases in the oral cavity. Thirst is a signal of water shortage in the body, commonly caused by salivary gland secretion disorder, mostly seen in the ageing change caused by aging, salivary secretion decreases, or after major surgery, radiotherapy affects salivary gland secretion, resulting in oral saliva decrease, often appearing dry mouth and thirsty symptoms, and is also the cause of rampant dental caries. Thirst may also be the manifestation of systemic disease in the mouth, such as the increase of sugar in the body will appear the symptom of thirst, drink a lot of water but still thirsty. This condition is mostly seen in patients with diabetes. Usually you should control your diet to keep your blood sugar stable. It may also be caused by fire, probably related to the stomach fire, usually should be a light diet, eat more vegetables and fruits, drink more water, replenish the body’s moisture, do not eat spicy and stimulating food, also related to staying up all night, affecting the normal metabolism, has a certain impact on the secretion of salivary glands, usually should be regular life, pay attention to rest do not stay up late, quit smoking and alcohol, maintain a reasonable exercise. So thirst may be a result of radiation therapy leading to salivary gland secretion disorder, or a manifestation of diabetes.