The composites were prepared by modifying silicon carbide fiber with particles of zirconium carbide (ZrC) and boron carbide (B4C) and incorporating them into a phenolic resin matrix. The influence of ZrC and B4C on the mechanical performance of SiCf/phenolic composites after high-temperature pyrolysis was studied through flexural performance test. The results show that the composite material has good thermal stability and high-temperature mechanical properties. After static ablation at 1 400 °C for 15 minutes, the flexural strength of the composite material reaches 286 MPa, which is still 7.3% higher than at room temperature, indicating that the composite material still has good mechanical properties even after heat treatment at 1 400 °C.