A preprandial blood glucose of 20 mmol/L on insulin suggests that diabetes is more dangerous, and should be alert to the presence of acute complications of diabetes such as diabetic ketoacidosis. Diabetic ketoacidosis is an acute complication of diabetes mellitus, clinically characterized by hyperglycemia, hyperketonemia and acidosis. Patients may present with symptoms such as nausea, vomiting, and a rotten apple odor in the exhaled breath. Ketone bodies may appear in the patient’s urine, and progression of the disease may result in water loss, shock, coma and other manifestations. If not treated in time, it may lead to death. If diabetic patients with preprandial blood glucose 20mmol / L without acute complications of diabetic ketoacidosis, but also due to the patient’s poor long-term control of blood glucose, will accelerate the process of diabetic complications such as diabetic nephropathy, diabetic retinopathy and other diabetic complications, which will seriously affect the patient’s quality of life and reduce the life expectancy of the patient. If the patient has a preprandial blood glucose of 20mmol/L, he should go to the hospital in time and be treated under the guidance of the doctor.