The structural characteristics of fiber‐reinforced plastic composites are usually determined by ablation, but bearing capacity is also a key parameter that should be clarified in aerospace engineering. Needle‐punched carbon/phenolic composites are most commonly used as the inner material of solid rocket motor nozzles, and the mechanical properties thereof are dynamic and difficult to obtain at high temperatures. A radiation‐heating mechanical testing machine with split‐test chambers was designed to test the in‐plane compression properties of the composite at different degrees or pyrolysis and temperatures. The results show that the final weight‐loss rate was 25.5%. During the loading process, the composite had a linear elastic mechanical response and brittle fracture failure, and the compressive strength was 218.9 MPa at room temperature. At different holding times, the higher the temperature, the higher the weight‐loss rate, the higher the graphitization degree of the specimen, and the lower the degree of disorder of the carbon matrix. Thus, the minimum compressive strength was found to be 23.42 MPa at 600°C, and the maximum was 60.16 MPa at 1800°C after high‐temperature pyrolysis. The present work provides a reference for the refined design of solid rocket motor nozzles.