Abstract: Based on the intrinsic characteristics of materials, the activation energy parameter can provide an alternatively feasible way as to achieve accurate assessment of the aging state and residual life of polymer insulations, however, accurate calculation of activation energy still remains a pending issue. According to equivalent approximation of the temperature integration, this paper proposed two kinds of transformation forms of the Arrhenius temperature integration, both of which render preferable applicability and accuracy. Based on the first transformation form of temperature integral, a thermal dynamic equation was established in term of the ratio of the heating rate, which is not only suitable for calculating the average activation energy of the reaction process, but also for that under non-isothermal conditions, without recurring to the reaction mechanism function. With the second transformation form of temperature integral being applied, the Flynn-Wall-Ozawa method was modified and improved in accuracy. Both methodologies were used to obtain the pyrolysis activation energy of the GIS insulator and the cross-linked polyethylene, which shows superior accuracy to the traditional Flynn-Wall-Ozawa method. Further research indicates that, the impact of low temperature approximation on activation energy calculation is negligible. The above proposed research presents effective methodologies in calculating the pyrolysis activation energy of polymer insulations.