The diagnosis of diabetes mellitus is based on the typical symptoms of “three more and one less” (excessive drinking, excessive urination, excessive eating, and weight loss) plus the measurement of venous plasma glucose ≥11.1 mmol/L, fasting blood glucose ≥7.0 mmol/L, or 2-hour glucose ≥11.1 mmol/L after glucose tolerance test at any time of the day, which can be diagnosed as diabetes mellitus if one of them is met. One of them is enough for a diagnosis of diabetes mellitus. The China Type 2 Diabetes 2020 Prevention and Control Guidelines include glycosylated hemoglobin in the diagnostic criteria, and glycosylated hemoglobin ≥6.5% can be used as an additional diagnostic criterion for diabetes, but diabetes cannot be ruled out even if it is lower than 6.5%. Patients diagnosed with diabetes should actively cooperate with doctors for treatment.