Vitamin D and pancreatic islet beta cells in diabetic patients have a reciprocal relationship because vitamin D acts as a chemical signal during the secretion of pancreatic islet beta cells, participating in cellular signaling and thus influencing the process of islet secretion. Vitamin D binds to receptors, regulates both PKA and PKC signaling pathways, increases intracellular calcium levels, stimulates insulin secretion, and it inhibits the production and release of reactive oxygen species clusters, interleukin-6, and tumor necrosis factor alpha in diabetic patients, decreasing lipotoxicity and glucotoxicity, thereby reducing apoptosis in pancreatic β-cells. For diabetic patients, especially type 1 diabetic patients, who are mostly combined with vitamin D deficiency, exogenous supplementation of vitamin D is beneficial to improve the pancreatic islet function of patients and facilitate blood glucose control.