C/SiBCN composites are expected to be widely used in aerospace applications because of their excellent high-temperature stability. However, the interfacial reactions have significantly limited their practical application. A pyrocarbon (PyC) interphase can improve the interfacial reactions of C/SiBCN composites. In this study, PyC interphases of different thicknesses (0.1 μm, 0.25 μm, and 0.5 μm) were introduced via chemical vapor deposition (CVD) process. The interface bonding of C/SiBCN composites with 0.1 μm and 0.25 μm thick interphases was relatively weak and the composites with 0.5 μm interphase exhibited strong interface bonding. After heat treatment at 1600 °C, the mechanical properties of the C/SiBCN composites with the 0.5 μm thick interphase was maintained at 131 MPa, and it was maintained at 105 MPa even after heat treatment at 1900 °C, indicating their excellent high-temperature mechanical properties. In short, 0.5 μm thick PyC interphase can effectively improve the interfacial reaction of the C/SiBCN composites, facilitating their application in high-temperature environments.