Provided are a glycosyltransferase, a mutant, and an in vitro glycosylation method. The method comprises the steps of: transferring a glycosyl group of a glycosyl donor to a hydroxyl group at the C-3 position of a tetracyclic triterpenoid compound in the presence of a glycosyltransferase to form a glycosylated tetracyclic triterpenoid compound, wherein the glycosyltransferase is a glycosyltransferase as shown in SEQ ID NO.:4 or a polypeptide derived therefrom, or a glycosyltransferase as shown in SEQ ID NO.:21 or a polypeptide derived therefrom.