C/SiC composite materials have the advantages of high temperature resistance, low expansion, and excellent thermal stability, and are widely used in aerospace, military, energy and other fields. The thermal conductivity of C/SiC composite materials is an important characteristic to ensure the stable operation of thermal protection structures in high-temperature environments. In order to investigate the effects of carbon fiber volume fraction and yarn porosity on the effective thermal conductivity of composite materials, a three-dimensional fine-knitted perforated C/SiC (3DP C/SiC) composite materials cell model was established. The in-plane and out of plane thermal conductivity of the composite materials were calculated. The results indicate that the out of plane thermal conductivity is smaller than the in plane thermal conductivity. The difference between in-plane thermal conductivity and out of plane thermal conductivity decreases as the external porosity of the yarn increases. Additionally, the influence of carbon fiber volume fraction and yarn porosity on the effective thermal conductivity of composite materials was numerically studied. The results indicate that as the volume fraction of carbon fiber and the porosity inside and outside the yarn increase, the in-plane and out of plane thermal conductivity of the composite materials both decrease. In addition, the effective thermal conductivity of 3DP C/SiC composite considering the pores in the yarn is lower than that without considering the pores in the yarn.