To further improve the mechanical properties of high-entropy carbides and high-entropy borides ceramic, in this study SiB₆ was introduced as a boron and silicon source into high-entropy carbide (Ti, Zr, Nb, Ta, Mo)C, and (Ti, Zr, Nb, Ta, Mo)B₂–(Ti, Zr, Nb, Ta, Mo)C based composites were synthesized via reactive spark plasma sintering at 2000 ℃. Research findings indicate that SiB₆ reacts with (Ti, Zr, Nb, Ta, Mo)C at high temperatures to form high-entropy boride (Ti, Zr, Nb, Ta, Mo)B₂, SiC and C phases. The densification of (Ti, Zr, Nb, Ta, Mo)B₂–(Ti, Zr, Nb, Ta, Mo)C based composites prepared with SiB₆ addition reaches 98.7%–99.7%. The grain size of high-entropy phases in composite ceramics with 10%–15% SiB₆ addition (volume fraction) is 0.84–0.92 μm, which is smaller than that of pure (Ti, Zr, Nb, Ta, Mo)C ceramics (~3.19 μm). Due to the fine grain strengthening, the hardness of (Ti, Zr, Nb, Ta, Mo)B₂–(Ti, Zr, Nb, Ta, Mo)C based composites (23.54–24.93 GPa) is higher than that of pure (Ti, Zr, Nb, Ta, Mo)C ceramics (~23.22 GPa). Additionally, the fracture toughness of (Ti, Zr, Nb, Ta, Mo)B₂–(Ti, Zr, Nb, Ta, Mo)C based composites increases with the addition of SiB₆, reaching up to 5.07 MPa·m^1/2, which is significantly higher than that of pure (Ti, Zr, Nb, Ta, Mo)C ceramics (3.02 MPa·m^1/2). Using (Ti, Zr, Nb, Ta, Mo)C and SiB₆ as raw materials and employing reactive spark plasma sintering, (Ti, Zr, Nb, Ta, Mo)B₂–(Ti, Zr, Nb, Ta, Mo)C based multiphase high-entropy ultra-high temperature ceramics with a fine-grained structure and excellent mechanical properties could be obtained.