A 3-hour postprandial blood glucose of 7.4 mmol/L may be abnormal, and normally the 3-hour postprandial blood glucose level will return to the fasting blood glucose level, i.e., 3.9-6.1 mmol/L. However, the 3-hour postprandial blood glucose is not used as a diagnostic criterion for diabetes mellitus, and is more often recognized as a random blood glucose. A 3-hour postprandial blood glucose of 7.4 mmol/L needs to be alerted to the possibility of diabetes. Diabetes mellitus is a metabolic disease characterized by elevated glucose levels in the blood; the typical clinical manifestations of diabetes mellitus are “three more and one less”, i.e., polyuria, polyphagia, polydipsia and weight loss. Long-term elevation of blood glucose may cause damage to the retina, kidneys, cardiovascular and other vital organs and tissues. The diagnosis of diabetes mellitus should be based on the combination of fasting plasma glucose, 2-hour postprandial plasma glucose, glycosylated hemoglobin, and the presence of the “three more and one less” symptoms. Patients diagnosed with diabetes mellitus should eat a light diet, exercise appropriately, take medication under the guidance of an endocrinologist, do not blindly use medication, and monitor their blood glucose regularly and follow up with an endocrinologist regularly to avoid delaying their condition.