Abstract: To meet the extreme thermal protection requirement of new-generation spacecrafts, nanoporous resin composites (IPC-90) with medium-density, high strength and excellent ablation/insulation properties had been prepared via a sol-gel polymerization using phenolic resin as nanoporous matrix and needled fiber fabric as the reinforcement. The effects of fiber type, namely quartz fiber (QF/IPC-90) and carbon fiber (CF/IPC-90) on the microstructure, mechanical properties, static thermal insulation, and ablation properties of the composites were systematically studied. The as-prepared IPC-90 with medium density of ~0.95 g/cm³ has excellent mechanical properties with tensile strength >120 MPa and bending strength >90 MPa. Due to the introduction of nanopore resin matrix and lightweight fiber felt, the resultant IPC-90 has relatively low room-temperature thermal conductivities (0.089 W/(m∙K) for QF/IPC-90 and 0.120 W/(m∙K) for CF/IPC-90), as well as low effective thermal conductivities at 1000℃. Furthermore, the possible ablation mechanisms under different temperatures were analyzed. It is found that both QF/IPC-90 and CF/IPC-90 have low linear ablation rates under the oxygen-propane ablation test below 2000℃, which are mainly caused by resin matrix pyrolysis and shrinkage. However, under the oxy-acetylene ablation test above 2000℃, the ablation of CF/IPC-90 is dominated by ultrahigh temperature carbonation-sublimation, while the severe ablation of CF/IPC-90 is caused by the melting of quartz fiber. Under the oxy-acetylene ablation of 4.2 MW/m², the linear ablation rates of CF/IPC-90 and QF/IPC-90 are 0.073 mm/s and 0.186 mm/s, respectively, being similar to the conventional high-density phenolic composites.