The normal range of fasting C-peptide for type 2 diabetes is 0.37-1.47nmol/L. The reference values are different for different kits and testing methods, so it is recommended to refer to the laboratory values in local hospitals. Insulin test may be affected by exogenous insulin, C peptide test can truly reflect the function of pancreatic islets, if the value of C peptide in patients with type 2 diabetes mellitus is lower than the normal range, it means that the function of pancreatic islets is impaired, and C peptide value is significantly higher than normal, it is possible that insulin resistance. Detecting changes in C-peptide value can reflect the function of pancreatic islets, if the C-peptide value exceeds or is lower than the normal range, it is necessary to adjust the treatment program under the guidance of the doctor in a timely manner.