The present invention provides a method of sugar-guided modifying a glycosylated polypeptide. First, a boronic acid group of a probe molecule and a sugar group of the glycosylated polypeptide form a first covalent bond. Next, an alkyne group of a modifying group and an azide group of the probe molecule form a second covalent bond by adding a promoter. As a result, the modifying group can be close to the glycosylated polypeptide. Then, the modifying group can bind to a nucleophilic residue that is near the sugar group, through a nucleophilic addition reaction. The method of the present invention can selectively modify a given site with the guidance of the sugar group.