The heat transfer coefficient, temperature distribution, and stress distribution of 3D carbon/carbon (C/C) composite solid rocket motor (SRM) throat at the thermal equilibrium state were calculated through the comprehensive thermo-structural simulation, and SRM used the tri-component propellant of ammonium perchlorate/hydroxyl-terminated polybutadiene/Al under the condition of 6.5 MPa for long run of 20, 60, and 95 s. The analysis of ablation behavior and mechanism of different throat regions was also conducted according to the simulation results. The results show that the temperature at throat convergent zone is the highest due to the thermochemical ablation between the oxidation components (H₂O, CO₂, H₂) and the surface carbon. The central throat zone has the maximum heat transfer coefficient with relatively high temperature. The ablation in the central throat zone is the most severe due to the stress of inner surface, including the gas thermochemical ablation, high-speed-flow mechanical denudation, and particle erosion. The central throat surface has small grooves or cracks, and the throat tends to decompose due to the stress and oxidation. The stress and temperature of throat exit zone are decreased, and the ablation rate is obviously reduced. The throat ablation mechanism is the combination effect of thermochemical ablation of oxidation components and the carbon influenced by the stress and temperature, flow mechanical denudation, and Al₂O₃ particle erosion.