Abstract: To address material ablation and surface recession of C/C composite nozzles in solid rocket motor operating under complex high-temperature two-phase gas flow environments, a dynamically coupled computational method was developed, which integrates gas-solid two-phase flow, radiative heat transfer from alumina particles and thermochemical ablation. The Oka model was separately adopted to evaluate the particle-induced mechanical erosion rate, while a finite-rate chemical kinetics model was used to calculate the thermochemical ablation rate, and dynamic mesh technique was employed to simulate nozzle profile recession during chemical ablation. The numerical results show that two-phase flow and radiative heat transfer significantly affect internal heat transfer in nozzle. In the convergent section, particle radiation attenuation reduces wall temperature and ablation rate, whereas in the divergent section, particle thermal inertia delays cooling and sustains continuous heat release, resulting in an opposite trend. Both thermochemical ablation and mechanical erosion mainly occur in upstream of the throat, and the thermochemical ablation rate rises with the increase of inlet temperature and pressure. Fine particles markedly mitigate the nozzle ablation rate, while particle with larger size and higher mass fractions substantially aggravate mechanical erosion in the convergent section. This method can effectively predict the internal thermal environment and transient ablation process of nozzle, which can provide reference for thermal protection design and performance optimization of solid rocket motor.