Abstract: Based on prototype experiment of a novel radial pintle-type variable-thrust SRM, severe ablation of the pintle profile and axial groove ablation on the divergent-section insulation layer behind the pintle were observed. The influence of the pintle on the two-phase flow of the nozzle and the boundary layer flow near the wall was analyzed via numerical simulation. Combined with the ablation phenomenon, ablation mechanisms of the radial pintle and the thermal insulation layer of the divergent section were revealed, and the thermal insulation material optimization and verification tests were conducted. The results show that the ablation on the windward surface of the pintle is primarily influenced by the enhanced heat transfer from gas jet impingement and the mechanical erosion of condensed-phase particles. Severe ablation occurs on the windward surface of the radial pintle, with a maximum linear ablation rate of 0.2 mm/s. The divergent section insulation mainly undergoes thermochemical ablation. The formation of the axial groove is attributed to the complex wave system structure in the flow field of the nozzle divergent section after introducing the radial pintle, which disturbs the boundary layer flow and forms a high-vorticity band at a specific location behind the radial pintle, thus aggravating the thermochemical ablation of the insulation. After replacing the carbon/phenolic material with a punctured C/C composite with a density of 1.86 g/cm³, the length of the axial ablation groove in the divergent section is reduced by 43%, the width is reduced by 7%, and the depth is reduced by 8%, which significantly improves the ablation morphology of the divergent section.