Carbon/carbon composites with higher mechanical strength and better reliability at elevated temperatures are urgently needed to satisfy the practical applications requirements. SiC nanowires (SiCNWs) modified C/C (SC-CC) composites have attracted an abundance of attention for their excellent mechanical performance. To further boost the mechanical strengths of composites and maximize the reinforcing efficiency of SiCNWs, we introduce orthogonally structured graphene nanosheets (OGNs) into SC-CC composites, in which OGNs are grafted on the SiCNWs via chemical vapor deposition (CVD) method, forming SC-G-CC composites. Benefiting from the nano-interface effects, uniform stress distribution, strong SiCNWs/PyC interfacial bonding and elevated stress propagation efficiency in the PyC matrix are achieved, thus SC-G-CC composites accomplish brilliant mechanical properties before and after 1,600 °C heat treatment. As temperature rises to 2,100 °C, SiCNWs lose efficacy, whereas OGNs with excellent thermal stability continue to play the nano-interface role in the PyC matrix. Therefore, SC-G-CC composites show better mechanical performance after 2,100 °C heat treatment, and the mechanical strength retention rate (MSR) of interlaminar shear strength, out-of-plane and in-plane compressive strength of SC-G-CC composites reach 61.0%, 55.7% and 55.3%, respectively. This work proposes an alternative thought for maximizing the potentiality of nanomaterials and edifies the mechanical modification of composites.