Abstract: Internal structure changes of Polyethersulfoneketone film during pyrolysis in argon from room temperature to 950℃ were studied by Fourier transform infrared spectroscopy (FTIR). From the variations of the spectral bands, it is suggested that the scission of the phthalazine ring along N-N bond occurs during pyrolysis, leading to the formation of compounds of phenyl with the conjugated nitrile group and the isocyan group. As the isocyan group is polymerized further, diphenyl carbondiimine may be formed, further polymerization of which forms a nitrogen-containing polycyclic aromatic compound. Further continuous carbonization should lead not only to a combining of the heteroaromatic ring, but also to the evolution of small gaseous molecules like hydrogen cyanide and the formation of a giant nitrogen-containing polycyclic compound, which can be further polycondensed to form a solid product with a giant polyaromatic planar ring.