A 20vol.%SiCw/ZK51A Mg-based composite was fabricated by the process of semi-solid extrusion directly following liquid infiltration. The microstructure and mechanical properties of the composite were investigated in comparison with its squeeze cast and hot extruded counterpart. The results showed that, by semi-solid extrusion, the SiC whiskers were well aligned and microstructure defects associated with the casting technology were eliminated. In addition, as compared with the conventional hot extrusion, the damage to the SiC whiskers during semi-solid extrusion was reduced significantly. Consequently, the composite fabricated by the semi-solid extrusion presented the best whisker reinforcing effect, with its elastic modulus, 0.2% offset yield strength, and ultimate tensile strength achieving 82.5GPa, 383.6MPa, and 431.8MPa respectively.