C/ C composites were prepared by chemical vapor deposition. During the process integral felt and propane were used as perform and carbon source separately. Emphases were focused on temperature and residual time. The microstructure was observed by polarizing microscope, and the exhausts were detected by gas chromatograph. A commercial software COMSOL was used to study the intermediate gases and reaction path. Results indicated that the as-prepared pyrocarbon was characterized by smooth laminar structure. H2,C2H2,C2H4,CH4 were majority of the intermediate gases during deposition. Macro-molecule and H2 are increased by increasing of the temperature. C2H4 is quickly decreased by increasing of the temperature. Macro-molecules with low C-H bended energy are easy to form free radicals and the contents would first increase then decrease with increase of residual time. Contents of macro-molecules with unsaturated structures changed slowly. C2H4 played an important role during the whole deposition process.