Abstract: Carbon/carbon (C/C) composite bolts exhibit high-temperature resistance, low density, and favorable thermal expansion compatibility, and show potential as fasteners for hypersonic vehicles, re-entry thermal protection structures, and high-temperature structural assemblies. In this study, three-dimensional needled C/C bolts were investigated. Mechanical testing was conducted to determine the effective load-bearing density range, and four bolt systems, namely C/C, C/C-SiC, C/C-ZrSi, and C/C-SiC-ZrSi, were prepared to evaluate the effects of SiC/ZrSi modification on pore structure, oxyacetylene ablation behavior, and post-ablation residual load-bearing capacity. The results show that the compressive strength of C/C composites increases from approximately 120 MPa to 175 MPa, while the flexural strength reaches 232.65 MPa at a density of about 1.7 g/cm³. The tensile strength of C/C bolts increases from approximately 17 MPa to 43.5 MPa as the density increases to about 1.5 g/cm³, and the double-shear strength increases from approximately 24 MPa to 50–60 MPa. Accordingly, 1.4-1.7 g/cm³ is identified as the effective load-bearing density range. Micro-CT results show that SiC modification reduces the porosity of C/C bolts from 11.87% to 1.77%, while the ZrSi-containing samples exhibit more discrete pore/defect distributions and lower connectivity. After oxyacetylene ablation, the mass ablation rate is 0.033 mg/s for C/C bolts and decreases to 0.013 mg/s for C/C-SiC bolts; the ZrSi-containing samples maintain relatively low mass ablation rates and more intact thread profiles. XRD, Raman, and SEM-EDS analyses show that ZrO₂-related products form on the ZrSi-containing samples after ablation, the relative fraction of defect-rich carbon in the residual carbon phase decreases, and Si, Zr, and O are detected on the ablated surfaces. The apparent tensile and double-shear strength retentions of unmodified C/C bolts are 62.65% and 94.34%, respectively, whereas those of C/C-ZrSi bolts reach 171.44% and 141.18%. SiC/ZrSi modification helps reduce pore/defect connectivity, preserve post-ablation thread profiles, and mitigate thread-root damage, thereby improving the apparent post-ablation residual load-bearing capacity of C/C bolts.