<p>Deciphering the cell-surface glycan codes through imaging relies on efficient covalent labeling and quantitative interpretation of the data. However, conventional methods are hampered by slow kinetics, and the conversion of covalent labeling-based imaging signals into biologically meaningful parameters remains a fundamental challenge. Here, we report a rapid live-cell hydrazone bioconjugation reaction for efficient covalent labeling of cell-surface sialic acids and galactose/<i>N</i>-acetylgalactosamine. We introduce a phenomenological kinetic model that quantitatively correlates membrane fluorescence intensity with reaction kinetics and local glycan density. This framework allows us to resolve cellular glycan fingerprints and intercellular glycan heterogeneity directly from imaging data. Capitalizing on the high labeling efficiency, we monitor the spatiotemporal dynamics of glycan density during macrophage M1 polarization. Our analysis reveals a critical discrepancy between confocal microscopy (reporting glycan density) and flow cytometry (reporting total abundance), underscoring the necessity of density measurement for understanding processes involving morphological changes. This work provides a robust chemical-analytical platform for quantitative glycan imaging, bridging a long-standing gap between interfacial reaction kinetics and biological discovery.</p>