Abstract: To meet the extreme thermal protection and load-bearing requirement of aerospace vehicle, three-dimensional orthogonal fiber reinforced nanoporous resin composites (3DIPC) have been prepared using three-dimensional quartz fiber preform as reinforcement and high-strength nanoporous phenolic resin as matrix. The as-prepared 3DIPC exhibit a mid-density of ~1.46 g·cm⁻³, low room-temperature thermal conductivity (<0.30 W·(m·K)⁻¹), low linear ablation rate (~0.15 mm·s⁻¹) and excellent mechanical properties with tensile strength >400 MPa, compressive strength >390 MPa, bending strength >300 MPa and interlaminar shear strength >30 MPa. By adjusting the yarn fineness in different directions, the effects of meso-structure variations in fiber preforms on the mechanical properties of 3DIPC were systematically studied. The results show increasing the fineness of the Z yarn can enhance the compressive modulus and interlaminar shear strength of composite materials, but it leads to a deterioration of tensile properties and compressive strength. Increasing the fineness of the warp yarn can improve the tensile and bending properties in the warp direction, but the tensile and flexural properties in the weft direction will be reduced. Finally, a mesoscale finite element model incorporating both surface and internal structures was established based on the actual morphology of 3DIPC. Combining with the progressive damage model of the composite material, the tensile failure behavior of the 3DIPC was simulated using the finite element software ABAQUS. The results show that the damage in 3DIPC initiates at the matrix of yarns and propagates to pure matrix and the fibers of yarns as the strain increases. The failure of 3DIPC under tensile loading in the warp and weft direction is dominated by the fracture of fibers of warp and weft yarns, respectively. Furthermore, the fracture of fibers of surface-Z yarns and surface-weft yarns is the primary cause of early-stage damage in 3DIPC under tensile loading in the weft direction.