Extrusion directly following vacuum infiltration is a special forming technique that combines the advantages of liquid metal infiltration and semisolid extrusion. The major advantages of this process are elimination of porosity and shrinkage, good surface finish, good dimensional accuracy, high strength to weight ratio and near net shaping. Magnesium matrix composites are fabricated usually through stirring casting, powder forming, injecting deposition, liquid metal infiltration or die casting at present time. However few investigations on magnesium matrix composite are conducted for the specific characteristics of magnesium alloy, such as high chemical activity and easy oxidation. The present paper is focused on Csf/Mg composites obtained via infiltration of porous short carbon fiber preform by liquid Magnesium. The complete experiment setup is designed and fabricated by ourselves, which include the forming molds, the unit for melting the magnesium, the unit for vacuuming and the monitoring and collecting system of forming process parameters. In this method the whole experiment setup is vacuumed firstly. Then the pressurized nitrogen is used to infiltrate the magnesium melt through a porous preform of short carbon fibers. After the infiltration completed, the punch of the press extrude the magnesium-infiltrated preform out of the forming die to form the tubes or bars. X-ray diffraction (XRD), optical and SEM microscopes were used to characterize the infiltration and the microstructure of fabricated composites. The compression test was used to characterize the mechanical properties of fabricated composites. The results show that the preform was infiltrated thoroughly by melt magnesium and the fabricated Csf/Mg composites have excellent mechanical properties compared with the magnesium alloys. Csf/Mg composites should be very promising candidates for automobile parts and portable electronic appliance parts in the future.