The development of a facile cysteine-directed S-glycosylation strategy would facilitate the intensive investigation of the effect of glycosylation on protein translational modification. Herein, we introduce glycosyl Bunte salt as an efficient glycosyl donor for site-selective peptide/protein modification. The coupling reaction with cysteine thiols under alkaline buffer conditions proceeded chemoselectively, delivering homogeneous glycoconjugates and inorganic salt as the only byproduct. A series of sugar moieties, including monosaccharides and oligosaccharides, were successfully conjugated to the peptides and protein via the disulfide bond. Furthermore, this protocol was applied to the glycosylation of the glucagon-like peptide 1 (GLP-1) variant, and its glycosylated effect on blood glucose control was also studied.