Structural color can be created when the ordered microstructure of an object interacts with incident light. Compared to the traditional pigment colors, it is bright, long-lasting, and environmentally friendly. As a result, it is of great significance for further research and development. Cellulose Nanocrystals (CNC) and silica are representatives of organic and inorganic particles that can be self-assembled to form structural color. Cellulose nanocrystals, originating from natural cellulose through sulfuric acid hydrolysis, can be stabilized in water through electrostatic repulsion owing to the sulfonic ester groups on their surface. At certain concentrations, colloidal solutions of these particles will self-assemble into cholesteric structures and retain them in dry films, resulting in structural colors caused by their optical activity. The periodic photonic crystal formed by the self-assembly of silica produces iridescent color attributed to the shielding effect of photonic band gap under incident light. At present, cellulose nanocrystals and silica are mainly combined with other materials respectively to form composites, but there are some shortages, such as the high cost of techniques or the complex appearance of optical properties. In order to solve the above problems, here we propose to fabricate the composite film with dual optical properties by self-assembly of the suspension of cellulose nanocrystals and polydisperse silica. Different optical performance was exhibited under natural light or point light source. In this study, the seed solution method of adding silica source step by step to control the hydrolysis-condensation rate of silica microspheres was used, resulting in the formation of polydisperse silica. The silica diameter ranges from 450 nm to 650 nm, which is distributed symmetrically around the mean diameter of 600 nm. Under a point light source, the photonic crystals formed by vertical self-assembly display iridescent color. Furthermore, the water dispersion of silica was blended with commercial nanocellulose solution, then composite films were fabricated by casting assembly. The effect of Polyvinylpyrrolidone (PVP) and Polyvinyl Alcohol (PVA) on the structural color was investigated. It is found that both PVP and PVA will make the structural color of cellulose nanocrystals red-shift and the silica iridescent color more brilliant. Better film formation is resulted with higher dosage of polyvinyl alcohol, while the film becomes less flexible with an increased dosage of PVP. When the aqueous dispersion of 0.3% (w/w) silica and 2% (w/w) cellulose nanocrystals are mixed in a solution mass ratio of 1∶4, with 1.2% PVP and 20%PVA, the better film-forming property is presented with dual optical properties. The film prepared here has both the chiral nematic structure of cellulose nanocrystals and the photonic crystal structure of silica_.The mutual superposition of structural colors will broaden its application in anti-counterfeiting, optical encryption, etc.