A mutant of EndoS2 includes one or more mutations in the sequence of a wild-type EndoS2 (SEQ ID NO:1), wherein the one or more mutations are in a peptide region located within residues 133-143, residues 177-182, residues 184-189, residues 221-231, and/or residues 227-237, wherein the mutant of EndoS2 has a low hydrolyzing activity and a high tranglycosylation activity, as compared to those of the wild-type EndoS2. A method for preparing an engineered glycoprotein using the mutant of EndoS2 includes coupling an activated oligosaccharide to a glycoprotein acceptor. The activated oligosaccharide is a glycan oxazoline.