The effect of pyrolysis on the chemical characteristics of Antibiotic Fermentation Residue (AFR) was studied in this paper. Electron Spin Resonance (ESR) spectrometry, X-ray photoelectron spectroscopy (XPS) and solid state ¹³C nuclear magnetic resonance (¹³C NMR) spectroscopy were applied to investigate the variation of free radicals and carbon structures. Results indicated that the free radical concentration (N_g) of AFR/chars changed dramatically during the pyrolysis process, first increasing and then decreasing. N_g reached 1.24 × 10¹⁹ spins per g in a N₂ atmosphere and 1.11 × 10¹⁹ spins per g in a CO₂ atmosphere at 320 °C. The g-values monotonically decreased, demonstrating that the chemical structures of free radicals changed during pyrolysis. Bond cleavage in methoxyl groups, aliphatic C–O bonds, aliphatic C–C bonds and CO groups was enhanced with increasing pyrolysis temperature. These carbon structures converted to aromatic C–O bonds and aromatic C–C bonds through polymerization and condensation reactions. Compared with the CO₂ atmosphere, the N₂ atmosphere was more conducive to the production of free radicals and aromatization of carbon structures in chars during pyrolysis process.