Diabetes mellitus (DM) is a metabolic disease that seriously threatens human health. In recent years, its prevalence has been increasing year by year worldwide. In order to reduce DM and its complications, early screening and diagnosis are particularly important. Glycosylated hemoglobin (HbA_1c) is an indicator of mean blood glucose concentration and plays an important role in blood glucose control and risk assessment of macrovascular and microvascular complications. HbA_1c was recognized by the World Health Organization as a diagnostic tool for DM in 2011. This paper introduces the history and research status of HbA_1c as a diagnosis indicator of DM, the value of HbA_1c in the screening and diagnosis of DM, and the influence of HbA_1c control on complications of DM.