Boron carbide possesses excellent properties and has a wide range of applications, but its production cost is relatively high. To address this issue, boron carbide-C composite powder was directly used, synthesized by carbothermal reduction method, as the raw material without any crushing and purification. Boron carbide composites were prepared through silicon infiltration reaction sintering, yielding material properties comparable to those prepared from commercially available boron carbide powder, which effectively reduced its preparation cost. This study mainly investigated the influence of the molar ratio of carbon to boron in the synthesis of powder on the phase composition, microstructure and properties of boron carbide ceramic composite materials, and explored the toughening mechanism of boron carbide ceramic composites. With the increase of the molar ratio of carbon to boron, the atomic ratio of carbon to boron in the synthesized boron carbide powder increases, as well as the content of free C, which coats the surface of the boron carbide particles at the molar ratio of 2.01. The phase composition of the boron carbide composite materials is B₁₂(C,Si,B)₃, SiC and Si. With the increase of carbon-boron molar ratio, the content of boron carbide and free Si in the composite decrease, while the content of SiC, the size of large SiC region, and the number of large SiC regions and SiC nanoparticles increase. Formation of large SiC regions decreases the strength and toughness of the material, while creation of SiC nanoparticles contributes to improvement of strength and toughness. When the carbon to boron molar ratio is 1.35, the composite exhibits the highest flexural strength and fracture toughness, reaching 338 MPa and 4.06 MPa∙m^1/2, respectively.