Calcium silicate hydrate (C?傆bS?傆bH) gel is a gene of cement-based cementitious materials, and its structural changes are crucial to the development of concrete macroscopic properties. To clarify the modification mechanism of cellulose nanocrystals (CNCs) in cement, C?傆bS?傆bH gels were prepared by a co-precipitation method, and the morphological and nucleation effects of CNCs on the microstructure of C?傆bS?傆bH gels were investigated by X-ray diffraction (XRD), transmission electron microscopy, nanoindentation, and nuclear magnetic resonance. The results show that the hydroxyl groups (-OH) on the surface of CNCs can complex Ca2+ ions, and then react with SiO42- ions in solution to form C?傆bS?傆bH gels that wraps around the CNCs. The CNCs can provide additional nucleation sites for the precipitation and growth of C?傆bS?傆bH gels, promote the reduction of the polymerization degree and the shortening of the chain length of the C?傆bS?傆bH gel, and increase the content of high-density C?傆bS?傆bH gel.