Peptides can be potent molecules with high efficacy and selectivity in the development of biotherapeutics. However, the poor pharmacokinetic properties of peptides pose major challenges for their broader medicinal applications. Inspired by the protein-stabilizing role of natural N-glycosylation, we design and synthesize a series of parathyroid hormone (PTH) peptides (1-34), bearing either N-GlcNAc or biantennary complex-type N-glycan modification, and evaluate their serum stability and biological activities. The results indicate that an N-Asn-linked complex-type sialylundecasaccharide can increase the serum half-life and in vivo bioactivity of PTH peptides with a broad tolerance of modification sites. Further, hydrogen/deuterium exchange mass spectroscopy indicates that the larger-sized N-glycan can induce enhanced hydration dynamics in its surroundings, which may facilitate an improved resistance for the peptide against enzymatic proteolysis. This sialylundecasaccharide-based peptide-engineering strategy has also been applied to glucagon-like peptide-1 (7-37), leading to glycopeptides with enhanced hypoglycemic activity and acting time in vivo. Together, these results demonstrate the potential of using sialylated complex-type N-glycan as a general engineering strategy for developing long-acting peptide therapeutics.