The synergistically enhanced polyurethane (PU) composites, in which the cellulose nanocrystal (CNC) was incorporated, were in situ polymerized and then the increasingly closer grid network structures for PU/CNC composites were achieved by adjusting chain extenders and adding different low contents of CNC, due to the introduction of different hard segments or the evolution of spherulites. As a result, the onset of the light transmission of PU elastomers and CNC‐filled composites showed an obvious red shift from UV light to visible light region, which meant that all UV light regions could be shielded entirely. Therefore, compared with Rhodamine B in water solution unprotected, the excellent UV‐light shielding was further verified by the inefficient photodegradation of Rhodamine B in water solution protected by PU‐PDA, PU‐BDI or PU‐PDA/CNC films under UV‐light irradiation. Meanwhile, enhanced light shielding behavior for PU‐PDA composites was observed with adding the CNC fillers. Moreover, the significantly enhanced mechanical properties were achieved, more than two or three times at elastic modulus, tensile strength and elongation at break. Finally, synergistically Enhanced mechanical properties in PU composites were found. Therefore, the study was of great significance in the potential light application. The microstructure of PU with different extenders and CNC contents resulted in different light properties.