Type 1 diabetes mellitus (T1DM) is still one of the major threats on global public health. This autoimmune condition is mainly caused by the imbalance of auto-reactive inflammatory effector T cells (Teffs) and protective regulatory T cells (Tregs). Therefore, inhibiting the development of Teffs and/or promoting Tregs provides a therapeutic strategy for preventing the development of T1DM. Pathways of energy metabolism have been shown to play a pivotal role in dictating the activation, differentiation and immune function of T cells. Studies have shown that inhibition of glycolysis suppresses the development of Th1 and Th17 cells, but promotes Treg production. AMP-activated protein kinase (AMPK) is a master sensor and regulator of cellular energy metabolism in mammals, which has also been demonstrated to interfere with T cell differentiation and effector function through inhibiting mammalian target of rapamycin (mTOR) and subsequent inhibition of glycolysis, and enhancement of lipid oxidation. In this study, we found that AMPK activator metformin suppresses T cell proliferation and inhibits the differentiation of Th1 and Th17 cells while promoting the development of Tregs in vitro in a dose-dependent manner. Treatment of NOD mice with metformin significantly mitigated autoimmune insulitis and substantially decreased the number of pro-inflammatory IFN-gamma+ as well as IL17+ CD4 T cells in the spleens of NOD mice. However, a significantly increased percentage of regulatory IL-10+ and Foxp3+ CD4 T cells were seen. We provided a novel potential therapeutic method-by regulating T cell metabolism through targeting AMPK, to reduce the severity of autoimmune insulitis.