The thermogravity/Fourier transform infrared (TG/FTIR) system was used to analyze the mechanism and kinetics of the thermal cracking of poly(ether ether ketone) (PEEK) under different environments. The thermal cracking of PEEK in a helium atmosphere showed that there were two stable cracking reaction regions. In the first-stage reaction, the thermal stability of the ketone group in PEEK was thermally more stable than the ether group. The cracking of the fluorenone structure in the carbonization showed it dominated the cracking scheme in the second-stage reaction. The thermal cracking of PEEK in air was governed by a random main-chain-scission and carbonization mechanism that increased with the cracking temperature under the influence of an increased thermal-oxidation mechanism. This mechanism predominated the PEEK solid reaction system until the compound completely was combusted. In air, the solid reaction rate of PEEK that produced a stable fluorenone structure was faster than that in helium and imparted a higher retarding effect on cracking in the initial lower temperature region. By means of the stable average activation energy for the kinetic parameter calculation for the order of reaction and pre-exponential factor, the theoretical TG curve was calculated and found to be identical to the observed TG curve. The kinetic model for thermal cracking of PEEK in air also is discussed.