Abstract: Carbon/carbon composites has been widely used in aerospace, weaponry and other fields with their excellent properties. However, the development of carbon/carbon composites has been limited by the high cost of carbon fiber preforms. Carbon foam has a three-dimensional network structure, and its ligaments show similar properties to carbon fibers, which can be used as the reinforcing phase to prepare carbon/carbon composites. In this paper, carbon foams with different carbon fiber volume contents (0vol%, 1vol%, 3vol%, 5vol%, 7vol%) were prepared as carbon/carbon composites preforms by using phenolic resin as the carbon source and NaCl as the pore-forming agent, and the carbon/carbon composites were prepared by using the rapid densification technique of thermal gradient chemical vapour infiltration (TG-CVI), which investigated the effects of carbon fiber content on the carbon fiber/carbon foam preform and its density, microstructure and mechanical properties after densification. The effects of carbon fiber content on the density, microstructure and mechanical properties of the carbon fiber/carbon foam preform and its densification were investigated. The results showed that with the increase of carbon fiber content, the number of microcracks in the carbon fiber/carbon foam precast body increased significantly, the density gradually decreased from 0.51 g/cm³ to 0.31 g/cm³, and the compressive strength decreased from 51.33 MPa to 1.35 MPa, flexural strength decreased from 42.53 MPa to 6.32 MPa. The compressive and flexural strengths of the carbon/carbon composites were significantly increased after densification, up to 183.67 MPa and 123.46 MPa, respectively, while the density was 1.09 g/cm³, resulting in high specific strength. The thermal conductivity of the composites increased from 0.298 W/(m·K) (before densification) to 2.484 W/(m·K), an increase of 734%, which was attributed to the formation of a three-dimensional thermal conductivity network between the carbon fibers and pyrolytic carbon after densification.