In this paper, a high-yield Hf-modified SiHfBOC ceramic precursor was developed, and a high-pressure assisted impregnation pyrolysis method was proposed to achieve the preparation of 3D PyC–C_f/SiHfBOC composites. This high-pressure assisted impregnation method significantly improves impregnation filling effect of the precursor in and between fiber bundles compared to dozens of traditional impregnation cycles. After undergoing just 9 precursor infiltration pyrolysis (PIP) cycles, the composites achieved relative density of approximately 90% and density of 1.64 g/cm³. The critical temperature difference of the 3D PyC–C_f/SiHfBOC composites after the shock of room temperature (RT)–1000 ℃ is as high as 650 ℃, which is twice that of traditional ceramic materials, showing good thermal shock resistance. Under the effect of Hf modification, a dense HfO₂–SiO₂ oxide layer (thickness of 93 μm) was formed in situ on the surface of the 3D PyC–C_f/SiHfBOC composites, effectively preventing further erosion of the composite matrix by high-temperature oxidation gas. Even in the ultra-high-temperature oxygen-containing environment at 1800 ℃, it still exhibits an excellent non-ablative result (with a linear ablation rate of 0.83×10⁻⁴ mm/s). This work not only enriches the basic research on lightweight ultra-high-temperature ceramic composites converted from Hf ceramic precursors, but also provides strong technical support for their applications in ultra-high-temperature non-ablative thermal protection materials for high-speed aircraft.