Carboxylated-chitooligomer (C-COS), which had a high moisture-absorption and moisture-retention ability, was prepared by oxidation of chitooligomer using a laccase/ 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) system. The intermolecular hydrogen bonding of C-COS was investigated by low-field nuclear magnetic resonance (LF-NMR), FTIR and 13CNMR, and its moisture-absorption and moisture-retention mechanism was deduced. The results showed that intermolecular hydrogen bonding were formed between polymer molecules and water in the course of C-COS hygroscopic process. More polar carboxylate ions on the C-COS molecule produced new hydrogen bonding effect. The decrease of the absorption peak intensity at 3363 and 1643 cm-1 and blue shift of the absorption frequency at the same wave number in the heating IR, as well as the increase of the absorption peak at 1643 cm-1 in the swelling IR, confirming that information of hydrated hydrogen bonds (H-O-H•••O-C=O) between water and carboxylate ions on the C-COS molecule. In addition, moisture-retention mechanism of C-COS was obtained that the polymer dissolved in the water system can form a huge polymer-water network structure, prompting the water molecule in the form of hydrogen bonds was firmly locked in this network structure.