It is necessary and urgent to develop high-performance flame retardants (FRs) with heat insulation and smoke suppression functions for public security. In this work, cellulose nanocrystal/alumina oxide (CNC/Al₂O₃) composites with corn-like structure were designed and synthesized by a facile sulphate half-esterification and chemical precipitation strategy. The hydrolyzed CNCs with rich sulfonate groups (SO₃^-) acted as carriers to anchor the Al₂O₃ nanoparticles. As the concentrations of Al³⁺ increased, denser Al₂O₃ nanoparticles were adsorbed on the surface of CNCs to form the protective layer, which significantly enhanced the thermal stability, heat resistance, and heat insulation of CNCs. Compared to CNCs, the maximum decomposition temperature (T_max) of the CNC/Al₂O₃-2 composite was increased by 85.0 °C, and the heat insulation efficiency was as high as 40.5%. More importantly, the formation of dense carbon layer on the surfaces significantly improved the flame retardancy and smoke suppression of CNC/Al₂O₃-2 composite under the synergistic protection of the Al2O3 nanoparticles layer.