Abstract: With the rapid development of China's domestic space engineering, the harsher requirements are put forward for lightweight, dimensional stability, thermal protection efficiency and long service capability of the thermal protection system. Rigid nanoporous phenolic resin-based RMI/PR composites are prepared via a sol-gel polymerization and ambient-pressure gradient drying using rigid mullite ceramic tile (RMI) as the reinforcement and hybrid phenolic resin (PR) as matrix. The effects of resin concentration on the microstructure, mechanical properties, thermal insulation properties and ablative properties of the composites are systematically studied. The results show that RMI has obvious transverse isotropy, and the room-temperature thermal conductivity in the Z direction is 0.036 W/(m∙K). With the increase of the resin concentration from 15wt% to 45wt%, the density of RMI/PR increases from 0.52 g/cm³ to 0.85 g/cm³, and the most probable pore size of the resin matrix decreases sharply from 2081 nm to 32 nm. With the increase of resin concentration, the room-temperature thermal conductivity of RMI/PR increases slowly and all of them are less than 0.07 W/(m∙K), but its mechanical properties are significantly enhanced and the maximum compressive strength in the Z direction of composites is up to 20.8 MPa. After static heat insulation test at 1000℃ for 300 s, the backside temperature of composites decreases from 277℃ to 240℃. Under the oxy-acetylene ablation at 2000℃ for 30 s, the linear ablation rate of the composites is reduced from 0.200 mm/s to 0.081 mm/s, indicating that the increase of resin concentration can significantly improve the high-temperature thermal insulation properties and ablation resistance of the composites.