Light-weight, heat-insulating and high-temperature resistant materials are essential for safety of astronauts and precision equipments. Nanocellulose (CNFs), with high specific surface area, low thermal expansion coefficient and high strength, has application prospects in preparation of light-weight and thermal insulation aviation materials. However, brittleness and high flammability of CNFs limit their widespread use in high-temperature fields. In order to improve the high temperature resistance of CNFs, [PMo₁₂O₄₀]^3- intercalated ZnAl-PMo₁₂O₄₀-LDHs (PMo-LDHs, LDHs: layered double hydroxides) were successfully synthesized by co-precipitation and ion-exchange methods. Then the PMo-LDHs+BA/CNFs aerogel was prepared by combining ZnAl-PMo₁₂O₄₀-LDHs with boric acid (BA) to formulate CNFs composites. When the mass fractions of PMo-LDHs and BA are 62.5% and 2.0% of CNFs, density and thermal conductivity of 62.5PMo-LDHs+BA/CNFs aerogel are 16.28 kg·m^-3 and 0.044 W/(m·K), respectively. Thermal insulation back-fire temperature experiments indicate that t₂₅₀ (time required to reach a thermal insulation temperature of 250 ℃) of 62.5PMo-LDHs+BA/CNFs aerogel is 2022.8 s, which is 867.8 s longer than that of pure CNFs aerogel. R₂₅₀ (heating rate when an insulation temperature reaches 250 ℃) is only 0.124 ℃·s^-1, which is 57.4% of R₂₅₀ for pure CNFs aerogel, demonstrating an exceptionally excellent thermal insulation effect. Combustion experiments reveal that the pure CNFs aerogel undergoes complete combustion within 15 s, whereas 62.5PMo-LDHs+BA/CNFs aerogel does not ignite within 81 s and exhibits no obvious shrinkage or deformation. Morphological observation of combustion residues indicates that a dense and uniform continuous carbon layer is formed on the surface of the aerogel due to the decomposition of PMo-LDHs, which significantly improves fire resistance of the CNFs aerogel.