Abstract: The microstructure and properties of C/C-SiC composites prepared by reactive melt infiltration (RMI) are significantly affected by post-heat treatment. In order to study the effect and mechanism of post-heat treatment on the microstructure and mechanical properties of C/C-SiC composites prepared by RMI, the isothermal chemical vapor infiltration (CVI) process was used to deposit pyrolytic carbon matrix in the carbon fiber preform, and C/C porous bodies with a density of 1.2 g/cm³ were prepared by using natural gas as carbon source gas and nitrogen as carrier gas and dilution gas. Then C/C-SiC composites were prepared by reactive melt infiltration method. The effects of different post-heat treatment temperatures on the phase composition, internal stress and mechanical properties of C/C-SiC composites were studied. The prepared C/C-SiC composites were treated at 1300℃, 1500℃ and 1700℃, respectively. The effects of post-high temperature heat treatment on the density, porosity, matrix composition, internal stress and bending properties of the C/C-SiC composites were investigated. The results show that after heat treatment at 1300℃, 1500℃ and 1700℃, the density of C/C-SiC composites decreases, the porosity increases, the content of SiC matrix increases, the distribution of SiC matrix becomes more extensive, and the residual Si content decreases significantly with large pores caused by residual Si volatilization. At 1300℃, 1500℃ and 1700℃, the bending strength increases first and then decreases. At 1500℃, the bending strength reaches a maximum of 296.52 MPa. With the increase of the post-treatment temperature, the bending modulus decreases, and at 1700℃, the bending strength decreases the most.