As a renewable and highly crystalline nano-object, chitin nanocrystals (ChNC) are the mechanical building blocks in the cell walls of fungi and the exoskeletons of shellfish, insects, and other crustaceans and have attracted intense interest in the production of functional materials. The structural feature of ChNC is composed of acetylglucosamine units with two hydroxyl groups and one acetyl group, and the latter can be transformed into more active amino groups by deacetylation. One issue is that obtaining more accessible amino groups by increasing the degree of deacetylation of ChNC under the preservation of the original crystalline property and rodlike morphology determines its subsequent modification and potential application. We analyze the influence of acid hydrolysis on the formed amino groups in the preparation process of ChNC and further investigate the tradeoff between gradiented deacetylation and varied crystalline index of this natural nanocrystal. The deacetylated ChNC obtained surface amino groups and retained its crystallinity, followed by surface fluorescence modification in an electrophilic addition reaction based on electrophilic addition reaction between the amino groups on its surface and the thiocyano groups of a fluorescent molecule. Finally, the fluorescent-modified ChNC is introduced into a water-borne polyurethane-based nail polish coating, providing a bright fluorescence performance, superior mechanical enhancement, and a rigid nanocomponent as a multifunctional additive.