The relationship between the characteristic structure and thermal stability of phenolic resin is of great significance for the selection of resin matrix for ablative materials. In this paper， the typical structure of phenolic resin was quantitatively described by ¹H nuclear magnetic resonance spectroscopy. The activation energy， reaction mechanism function， and thermal stability of different typical structured phenolic resins were studied. The results show that the activation energy of phenolic resin with different structures is 162 to 240 kJ/mol. The activation energy of the pyrolysis reaction is the highest and the thermal stability of the resin is optimum when the F/P value is 1.2 to 1.5. The most probable mechanism function of pyrolysis at 570 ℃ is g（a）=（1-α）^-b-1（b=5.2~6.7）， which is the chemical reaction mechanism. The results of pyrolysis kinetics are verified by qualitative and quantitative analysis of pyrolysis gas products using Py-GC/MS.