Vascularized islets and their preparation method is presented. The vascularized islets are comprised of islet spheroids enclosed within a network of blood vessels. The vascular network is composed of endothelial cells surrounded by smooth muscle cells, forming a tubular vessel with branched structures. The presence of this vasculature significantly enhances the survival of the islets, improves the efficiency of pancreatic precursor cells, islet progenitor cells and R cells, and the increases the synthesis of insulin precursor C-peptide. The procedures for constructing vascularized islets are suitable for long-term in vitro culture of islets.