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Aerogels with low density and excellent thermal insulation are seriously plagued by poor mechanical properties with low compressive moduli in the kPa range, especially in extreme conditions such as high temperatures and fires. In this work, a new double chemical cross-linking and physical cross-linking strategy is developed to synthesize ultrahigh-modulus aerogels with extreme-condition resistance from low-cost polyvinyl alcohol (PVA), bio-based phytic acid (PA) and montmorillonite (MMT). The resultant aerogels consist of interpenetrating networks with chemically cross-linked domains between PVA and PA and physically cross-linked domains among PVA, PA and MMT, leading to an ultrahigh compressive modulus of 41.9 MPa and a specific modulus of 455.4 MPa·cm3·g−1, which can even withstand a car crush. Benefitting from the formation of ultra-strong network backbones containing phosphorus and clay, the aerogels amazingly show excellent fire resistance and high-temperature thermal insulation features. This robust porous material can maintain its structural integrity and high modulus (~2 MPa) after exposure to extremely high-temperature flame (~1300 °C), and also exhibits high mechanical modulus of ~4 MPa after heating treatment at 900 °C. These special characteristics make the aerogel a promising insulation candidate in the fields of aviation, aerospace and other applications.