The work aims to prepare chiral nematic CNC/glycerin composite films with different optical properties by blending cellulose nanocrystals (CNC) suspension with glycerol for packaging anti-counterfeiting. CNC was prepared by sulfuric acid hydrolysis method, and its particle size, microstructure, polarizing properties, maximum reflection wavelength and thermogravimetric properties of the films were determined. CNC/glycerol composite films were prepared by blending CNC suspension with glycerol in different proportions by evaporation drying self-assembly method, and the optical properties and humidity sensitive properties of the composite films were measured. The results showed that the average particle size of cellulose nanocrystals was 220 nm, zeta potential value was −32 mV, and crystallinity was 81.88%. After drying, CNC suspension had bright rainbow color and birefringence effect under polarized light microscope. The cross section of the film had cholesteric liquid crystal layer structure with pitch of 178 nm. With the increase of glycerol content from 0.2 g to 1.2 g, the maximum reflection wavelength of the composite membrane shifted from 200 nm to 620 nm, and the wavelength red shift was obvious. CNC/glycerin composite film is sensitive to water rapidly, and the color of dry composite film changes reversibly when it meets water. Glycerol can control the reflection wavelength of cellulose nanocrystalline film in a certain range, and has the ability of fast color reversible humidity sensitive response.