Abstract: Relationship of densities between C_f/SiC composites and C/C preforms was studied by theoretical calculation. And 2D C_f/SiC composites were prepared from three kinds of C/C preforms with densities of 0.98, 1.06, 1.12 g/cm³, which were prepared by repeatedly overlapping the layers of carbon cloths and precursor infiltration pyrolysis, by liquid silicon infiltration. The effects of densities of C/C preforms on structure and property of composites were study. The results were compared with theoretical calculation results. The research results show that the test results are basically consistent with the theoretical mathematical results. The density of C_f/SiC composites increases first and then decreases with the increase of the C/C preforms density. And when the density of C/C preform is greater than 0.98 g/cm³, the flexural strength of composites decreases with the increase of C/C preforms density. The composites have a better structure and property, while the density of C/C preforms is 0.98 g/cm³.