This paper uses chemical vapor infiltration method to prepare 2.5D C/C-SiC composites, and conducts various tests to reveal its high-temperature mechanical properties and failure mechanism. The results show that the test temperature significantly affects the mechanical properties. As the test temperature increased from room temperature to 800 °C, the tensile strength of the 2.5D C/C-SiC composite dropped from 127.67 ± 20.87 MPa to 94.74 ± 28.15 MPa, and the performance dropped by 25.79 %. The compressive strength dropped from 326.92±17.37 MPa to 192.31 ± 29.15 MPa, resulting in a performance decrease of 41.18 %. The bending strength dropped from 355.74 ± 40.80 MPa to 235.88 ± 23.52 MPa, and the performance dropped by 33.69 %; the interlaminar shear strength dropped from 9.77 ± 2.79 MPa to 7.80 ± 3.26 MPa, and the performance dropped by 20.16 %. High-temperature oxidation will lead to interface degradation, destroy the composite matrix and carbon fiber structure, change the interface bonding strength, and ultimately lead to a decline in composite performance.