Morbidity and mortality of coronary artery disease associated with type 2 diabetes mellitus (T2DM) are in a rising trend. Some related evidence shows that insulin itself may have a dual role in this situation, and insulin is now still considered as a controversial role in T2DM patients due to its contribution to coronary atherosclerotic lesions. In recent years, results from some major observational studies are not consistent with those of randomized controlled trials. Nowadays, the “dual signal pathway hypothesis” of insulin is more popular in the mechanism of insulin in T2DM accompanied with coronary heart disease. It is of great importance to further elucidate the mechanism of insulin in T2DM with coronary heart disease.