Accurately modeling the kinetics of the pyrolysis reaction of phthalonitrile resin was helpful to investigate the pyrolysis mechanisms. To study the pyrolysis kinetics, phthalonitrile resin was measured by thermogravimetric analysis (TGA) at different heating rates in flowing nitrogen. The results indicated that there were two consecutive and overlapping stages in the pyrolysis process. The overlapped peaks were separated by multi-peak fitting method. Two models, namely, Kissinger, Kissinger-Akahira-Sunose (KAS), were employed to calculate activation energy at various stages of pyrolysis process. Kinetic models were determined using the differential method and Coats-Redfern method. Based on the attained kinetic models and activation energy values, the pre-exponential factor values were determined by KAS method. According to the calculated apparent activation energies, the most probable kinetic mechanism functions and pre-exponential factors, the simulated data agreed well with the experimental data.