Mathematical modeling for the preparation of C/SiC composites from methyl-trichlorosilane in a F-CVI(Forced-chemical vapor infiltration)reactor was studied. Changes of pressure, concentration and porosity with time were predicted by the modeling. Pressure in the preform decreased sharply along the direction of gas flow. Pore entrances were plugged at 130min reaction time in the conditions of thes research. As pore entrances became plugged, the pressure at the pore entrance increased rapidly the time when the preform should be overturned in the middle of deposition process for a uniform deposition could be decided by observing the radius of pore entrance. At the gas outlet of the preform, MTS was depleted completely and the fraction of HCI, i.e., the undesirable byproduct, became 0.42.