The surface ablation, the pyrolysis expansion and the linear expansion of thermal protection layer are the key factors for the thickness variation of thermal protection layer and internal temperature field. The surface ablation is widely discussed, while the other two factors are rarely involved. The ablation and internal temperature field for the phenalic-SiO₂ thermal protection materials are numerically simulated, which indicated that pyrolysis expansion and linear expansion are very important to both the thickness variation of thermal protection layer and the internal temperature field. Both the characteristics of the aerodynamic heating and the effect of the uncertainty of material properties are investigated, and the results shown that the further detail research on pyrolysis expansion phenomena is needed.