Abstract: Sialic acid transferase belongs to the family of glycosyl transferase, with cytidyl sialoside phosphate (CMP-Neu5Ac) as the donor substrate and transferring sialic acid to the ends of glycoproteins and glycolipids. Such enzymes are essential in the synthesis pathways of sialic acid oligosaccharides. Microbial sialyltransferases have broader substrate specificities and are readily overexpressed in common prokaryotic expression systems. It is of great significance for studying enzymatic properties and producing different sialyl oligosaccharides. This review summarized the classification and microbial sources of sialyltransferases, the crystal structure, the mechanism of catalytic reaction, the enzymatic properties, the clone and expression, and their applications in the synthesis of sialylated oligosaccharides. The future study direction of sialyltransferases was also prospected, in order to expand the application of these enzymes in sugar chemistry and sugar biology.