Castor oil-based polyurethane bionanocomposites with improved mechanical properties were prepared by the introduction of crab chitin nanocrystals from partial surface acetylation.Through the controlled acetylation, one part of hydrophilic hydroxyl groups on nanocrystals were replaced by hydrophobic acetyl groups in order to enhance the compatibility between chitin nanoparticles and polyurethane; and other part of hydroxyl groups were preserved on the surface of nanocrystals for the purpose of rigid network formation among chitin nanoparticles.The presence of acetylated chitin nanocrystals at the moderate concentration (6 wt%) will significantly promote the nano-reinforcing effect in composites, and simultaneously improved the strength, stiffness and toughness of thermoplastic polyurethane-based nanocompcsites.Meanwhile, derived from the restriction of rigid nanocrystals to soft segments, the glass transition temperatures of nanocomposites surprisingly increased with the higher loading levels of acetylated chitin nanocrystals.More importantly, this study provided a strategy for the discussion of synergy effects and "tradeoff" of adequate dispersion, network formation and interfacial adhesion of rigid nanoparticles in soft polymeric matrices, by means of structure and properties analysis of semi-transparent polyurethane/acetylated chitin nanocrystals composites.